Multi-Channel Electro-Optic Receiver with Polarization Diversity and Timing-Skew Management

ABSTRACT

An electro-optic receiver includes a polarization splitter and rotator (PSR) that directs incoming light having a first polarization through a first end of an optical waveguide, and that rotates incoming light from a second polarization to the first polarization to create polarization-rotated light that is directed to a second end of the optical waveguide. The incoming light of the first polarization and the polarization-rotated light travel through the optical waveguide in opposite directions. A plurality of ring resonators is optically coupled the optical waveguide. Each ring resonator is configured to operate at a respective resonant wavelength, such that the incoming light of the first polarization having the respective resonant wavelength optically couples into said ring resonator in a first propagation direction, and such that the polarization-rotated light having the respective resonant wavelength optically couples into said ring resonator in a second propagation direction opposite the first propagation direction.

CLAIM OF PRIORITY

This application is a divisional application under 35 U.S.C. 121 ofprior U.S. patent application Ser. No. 17/353,776, filed on Jun. 21,2021, which claims priority under 35 U.S.C. 119(e) to U.S. ProvisionalPatent Application No. 63/043,774, filed on Jun. 24, 2020. Thedisclosure of each above-identified patent application is incorporatedherein by reference in its entirety for all purposes.

BACKGROUND 1. Field of the Invention

The present invention relates to optical data communication.

2. Description of the Related Art

Optical data communication systems operate by modulating laser light toencode digital data patterns. The modulated laser light is transmittedthrough an optical data network from a sending node to a receiving node.The modulated laser light having arrived at the receiving node isde-modulated to obtain the original digital data patterns. Therefore,implementation and operation of optical data communication systems isdependent upon having reliable and efficient devices for modulatingoptical signals and for receiving optical signals. It is within thiscontext that the present invention arises.

SUMMARY

In an example embodiment, an electro-optic receiver is disclosed. Theelectro-optic receiver includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light having afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic receiver also includes anoptical waveguide having a first end optically to the first opticaloutput of the polarization splitter and rotator. The optical waveguidehas a second end optically connected to the second optical output of thepolarization splitter and rotator, such that the first portion of theincoming light travels from the first optical output of the polarizationsplitter and rotator through the optical waveguide in a first direction,and such that the polarization-rotated second portion of the incominglight travels from the second optical output of the polarizationsplitter and rotator through the optical waveguide in a second directionopposite the first direction. The electro-optic receiver also includes aplurality of ring resonator photodetectors positioned alongside theoptical waveguide and within an evanescent optical coupling distance ofthe optical waveguide. Each of the plurality of ring resonatorphotodetectors is configured to operate at a respective resonantwavelength, such that the first portion of the incoming light having awavelength substantially equal to the respective resonant wavelength ofa given one of the plurality of ring resonator photodetectors opticallycouples into the given one of the plurality of ring resonatorphotodetectors in a first propagation direction, and such that thepolarization-rotated second portion of the incoming light having awavelength substantially equal to the respective resonant wavelength ofthe given one of the plurality of ring resonator photodetectorsoptically couples into the given one of the plurality of ring resonatorphotodetectors in a second propagation direction opposite the firstpropagation direction.

In an example embodiment, an electro-optic receiver is disclosed. Theelectro-optic receiver includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light having afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic receiver also includes anoptical waveguide that has a first end optically to the first opticaloutput of the polarization splitter and rotator. The optical waveguidehas a second end optically connected to the second optical output of thepolarization splitter and rotator, such that the first portion of theincoming light travels from the first optical output of the polarizationsplitter and rotator through the optical waveguide in a first direction,and such that the polarization-rotated second portion of the incominglight travels from the second optical output of the polarizationsplitter and rotator through the optical waveguide in a second directionopposite the first direction. The electro-optic receiver also includes aplurality of ring resonators positioned alongside the optical waveguideand within an evanescent optical coupling distance of the opticalwaveguide. Each of the plurality of ring resonators is configured tooperate at a respective resonant wavelength, such that the first portionof the incoming light having a wavelength substantially equal to therespective resonant wavelength of a given one of the plurality of ringresonators optically couples into the given one of the plurality of ringresonators in a first propagation direction, and such that thepolarization-rotated second portion of the incoming light having awavelength substantially equal to the respective resonant wavelength ofthe given one of the plurality of ring resonators optically couples intothe given one of the plurality of ring resonators in a secondpropagation direction opposite the first propagation direction. Theelectro-optic receiver also includes a plurality of photodetectorsrespectively associated with the plurality of ring resonators. Theelectro-optic receiver also includes a plurality of output opticalwaveguides respectively optically coupled to the plurality of ringresonators. Each of the plurality of output optical waveguides includesa coupling section, a short section, and a long section. The couplingsection is positioned to evanescently couple light from a correspondingone of the plurality of ring resonators. The short section extends froma first end of the coupling section to a corresponding one of theplurality of photodetectors. The long section extends from a second endof the coupling section to the corresponding one of the plurality ofphotodetectors.

In an example embodiment, a method is disclosed for operating a photonicintegrated circuit. The method includes receiving incoming light throughan optical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightthrough a first end of an optical waveguide. The method also includesrotating the second polarization of the second portion of the incominglight to the first polarization so that the second portion of theincoming light is a polarization-rotated second portion of the incominglight. The method also includes directing the polarization-rotatedsecond portion of the incoming light through a second end of the opticalwaveguide. The optical waveguide extends in a continuous manner from thefirst end to the second end. The method also includes operating aplurality of ring resonators to evanescently in-couple light from theoptical waveguide. Each of the plurality of ring resonators is operatedat a respective resonant wavelength to in-couple both the first portionof the incoming light having the respective resonant wavelength and thepolarization-rotated second portion of the incoming light having therespective resonant wavelength.

In an example embodiment, an electro-optic receiver is disclosed. Theelectro-optic receiver includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light having afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic receiver also includes a firstoptical waveguide optically connected to the first optical output of thepolarization splitter and rotator. The electro-optic receiver alsoincludes a first plurality of ring resonators positioned within anevanescent optical coupling distance of the first optical waveguide.Each of the first plurality of ring resonators is configured to operateat a respective resonant wavelength, such that the first portion of theincoming light having a wavelength substantially equal to the respectiveresonant wavelength of a given one of the first plurality of ringresonators optically couples into the given one of the first pluralityof ring resonators. The electro-optic receiver also includes a firstplurality of output optical waveguides respectively positioned within anevanescent optical coupling distance of the first plurality of ringresonators. The electro-optic receiver also includes a second opticalwaveguide optically connected to the second optical output of thepolarization splitter and rotator. The electro-optic receiver alsoincludes a second plurality of ring resonators positioned within anevanescent optical coupling distance of the second optical waveguide.Each of the second plurality of ring resonators is configured to operateat a respective resonant wavelength, such that the polarization-rotatedsecond portion of the incoming light having a wavelength substantiallyequal to the respective resonant wavelength of a given one of the secondplurality of ring resonators optically couples into the given one of thesecond plurality of ring resonators. The electro-optic receiver alsoincludes a second plurality of output optical waveguides respectivelypositioned within an evanescent optical coupling distance of the secondplurality of ring resonators. The electro-optic receiver also includes aplurality of photodetectors. Each of the plurality of photodetectors isoptically connected to receive light from a respective one of the firstplurality of output optical waveguides and from a respective one of thesecond plurality of output optical waveguides, where the respective oneof the first plurality of output optical waveguides is optically coupledto one of the first plurality of ring resonators having a given resonantwavelength, and where the respective one of the second plurality ofoutput optical waveguides is optically coupled to one of the secondplurality of ring resonators having substantially the same givenresonant wavelength.

In an example embodiment, a method is disclosed for operating a photonicintegrated circuit. The method includes receiving incoming light throughan optical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightinto a first optical waveguide. The method also includes rotating thesecond polarization of the second portion of the incoming light to thefirst polarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light into a second optical waveguide. The method also includesoperating a first plurality of ring resonators to evanescently in-couplelight from the first optical waveguide. Each of the first plurality ofring resonators is operated at a respective resonant wavelength toin-couple light having the respective resonant wavelength from the firstoptical waveguide. The method also includes optically coupling lightfrom the first plurality of ring resonators into respective ones of afirst plurality of output optical waveguides. The method also includesdirecting light within the first plurality of output optical waveguidesinto respective ones of a plurality of photodetectors. The method alsoincludes operating a second plurality of ring resonators to evanescentlyin-couple light from the second optical waveguide. Each of the secondplurality of ring resonators is operated at a respective resonantwavelength to in-couple light having the respective resonant wavelengthfrom the second optical waveguide. The method also includes opticallycoupling light from the second plurality of ring resonators intorespective ones of a second plurality of output optical waveguides. Themethod also includes directing light within the second plurality ofoutput optical waveguides into respective ones of the plurality ofphotodetectors.

In an example embodiment, an electro-optic receiver is disclosed. Theelectro-optic receiver includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light having afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct apolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic receiver also includes a firstoptical waveguide that has a first end and second end. The first end ofthe first optical waveguide is optically connected to the first opticaloutput of the polarization splitter and rotator. The electro-opticreceiver also includes a second optical waveguide that has a first endand second end. The first end of the second optical waveguide isoptically connected to the second optical output of the polarizationsplitter and rotator. The electro-optic receiver also includes atwo-by-two optical splitter that has a first optical input opticallyconnected to the second end of the first optical waveguide. Thetwo-by-two optical splitter has a second optical input opticallyconnected to the second end of the second optical waveguide. Thetwo-by-two optical splitter has a first optical output and a secondoptical output. The two-by-two optical splitter is configured to outputsome of the first portion of the incoming light and some of thepolarization-rotated second portion of the incoming light through eachof the first optical output and the second optical output of thetwo-by-two optical splitter. The electro-optic receiver also includes athird optical waveguide optically connected to the first optical outputof the two-by-two optical splitter. The electro-optic receiver alsoincludes a first plurality of ring resonators positioned within anevanescent optical coupling distance of the third optical waveguide.Each of the first plurality of ring resonators is configured to operateat a respective resonant wavelength, such that light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the first plurality of ring resonators optically couples from thethird optical waveguide into the given one of the first plurality ofring resonators. The electro-optic receiver also includes a firstplurality of output optical waveguides respectively positioned within anevanescent optical coupling distance of the first plurality of ringresonators. The electro-optic receiver also includes a fourth opticalwaveguide optically connected to the second optical output of thetwo-by-two optical splitter. The electro-optic receiver also includes asecond plurality of ring resonators positioned within an evanescentoptical coupling distance of the fourth optical waveguide. Each of thesecond plurality of ring resonators is configured to operate at arespective resonant wavelength, such that light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the second plurality of ring resonators optically couples from thefourth optical waveguide into the given one of the second plurality ofring resonators. The electro-optic receiver also includes a secondplurality of output optical waveguides respectively positioned within anevanescent optical coupling distance of the second plurality of ringresonators. The electro-optic receiver also includes a plurality ofphotodetectors. Each of the plurality of photodetectors is opticallyconnected to receive light from a respective one of the first pluralityof output optical waveguides and from a respective one of the secondplurality of output optical waveguides, where the respective one of thefirst plurality of output optical waveguides is optically coupled to oneof the first plurality of ring resonators having a given resonantwavelength, and wherein the respective one of the second plurality ofoutput optical waveguides is optically coupled to one of the secondplurality of ring resonators having the same given resonant wavelength.

In an example embodiment, a method is disclosed for operating a photonicintegrated circuit. The method includes receiving incoming light throughan optical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightthrough a first optical waveguide and into a first optical input of atwo-by-two splitter. The method also includes rotating the secondpolarization of the second portion of the incoming light to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light through a second optical waveguide and into a secondoptical input of the two-by-two splitter. The method also includesdirecting some of the first portion of the incoming light through afirst optical output of the two-by-two optical splitter and into a thirdoptical waveguide. The method also includes directing some of the firstportion of the incoming light through a second optical output of thetwo-by-two optical splitter and into a fourth optical waveguide. Themethod also includes directing some of the polarization-rotated secondportion of the incoming light through the first optical output of thetwo-by-two optical splitter and into the third optical waveguide. Themethod also includes directing some of the polarization-rotated secondportion of the incoming light through the second optical output of thetwo-by-two optical splitter and into the fourth optical waveguide. Themethod also includes operating a first plurality of ring resonators toevanescently in-couple light from the third optical waveguide. Each ofthe first plurality of ring resonators is operated at a respectiveresonant wavelength to in-couple light having the respective resonantwavelength from the third optical waveguide. The method also includesoptically coupling light from the first plurality of ring resonatorsinto respective ones of a first plurality of output optical waveguides.The method also includes directing light within the first plurality ofoutput optical waveguides into respective ones of a plurality ofphotodetectors. The method also includes operating a second plurality ofring resonators to evanescently in-couple light from the fourth opticalwaveguide. Each of the second plurality of ring resonators is operatedat a respective resonant wavelength to in-couple light having therespective resonant wavelength from the fourth optical waveguide. Themethod also includes optically coupling light from the second pluralityof ring resonators into respective ones of a second plurality of outputoptical waveguides. The method also includes directing light within thesecond plurality of output optical waveguides into respective ones ofthe plurality of photodetectors.

In an example embodiment, an optical input polarization managementdevice is disclosed. The optical input polarization management deviceincludes a polarization splitter and rotator that has an optical inputoptically connected to receive incoming light. The polarization splitterand rotator has a first optical output and a second optical output. Thepolarization splitter and rotator is configured to direct a firstportion of the incoming light that has a first polarization through thefirst optical output. The polarization splitter and rotator isconfigured to rotate a polarization of a second portion of the incominglight from a second polarization to the first polarization so that thesecond portion of the incoming light is a polarization-rotated secondportion of the incoming light. The polarization splitter and rotator isconfigured to direct the polarization-rotated second portion of theincoming light through the second optical output. The optical inputpolarization management device also includes a first optical waveguidethat has a first end and second end. The first end of the first opticalwaveguide is optically connected to the first optical output of thepolarization splitter and rotator. The optical input polarizationmanagement device also includes a second optical waveguide that has afirst end and second end. The first end of the second optical waveguideis optically connected to the second optical output of the polarizationsplitter and rotator. The optical input polarization management devicealso includes a first phase shifter interfaced with either the firstoptical waveguide or the second optical waveguide. The optical inputpolarization management device also includes a first two-by-two opticalsplitter that has a first optical input optically connected to thesecond end of the first optical waveguide. The first two-by-two opticalsplitter has a second optical input optically connected to the secondend of the second optical waveguide. The first two-by-two opticalsplitter has a first optical output and a second optical output. Theoptical input polarization management device also includes a thirdoptical waveguide that has a first end and second end. The first end ofthe third optical waveguide is optically connected to the first opticaloutput of the first two-by-two optical splitter. The optical inputpolarization management device also includes a fourth optical waveguidethat has a first end and second end. The first end of the fourth opticalwaveguide is optically connected to the second optical output of thefirst two-by-two optical splitter. The optical input polarizationmanagement device also includes a second two-by-two optical splitterthat has a first optical input optically connected to the second end ofthe third optical waveguide. The second two-by-two optical splitter hasa second optical input optically connected to the second end of thefourth optical waveguide. The second two-by-two optical splitter has afirst optical output and a second optical output. The optical inputpolarization management device also includes a second phase shifterinterfaced with either the third optical waveguide or the fourth opticalwaveguide. The optical input polarization management device alsoincludes a fifth optical waveguide optically connected to either thefirst optical output of the second two-by-two optical splitter or thesecond optical output of the second two-by-two optical splitter.

In an example embodiment, a method is disclosed for optical inputpolarization management. The method includes receiving incoming lightthrough an optical input port. A first portion of the incoming light hasa first polarization, and a second portion of the incoming light has asecond polarization. The method also includes splitting the firstportion of the incoming light from the second portion of the incominglight. The method also includes directing the first portion of theincoming light through a first optical waveguide and into a firstoptical input of a first two-by-two splitter. The method also includesrotating the second polarization of the second portion of the incominglight to the first polarization so that the second portion of theincoming light is a polarization-rotated second portion of the incominglight. The method also includes directing the polarization-rotatedsecond portion of the incoming light through a second optical waveguideand into a second optical input of the first two-by-two splitter. Themethod also includes operating a first phase shifter interfaced witheither the first optical waveguide or the second optical waveguide toapply a controlled amount of shift to a phase of light traveling througheither the first optical waveguide or the second optical waveguide towhich the phase shifter is interfaced. The method also includesdirecting some of the first portion of the incoming light through afirst optical output of the first two-by-two optical splitter and into athird optical waveguide. The method also includes directing some of thefirst portion of the incoming light through a second optical output ofthe first two-by-two optical splitter and into a fourth opticalwaveguide. The method also includes directing some of thepolarization-rotated second portion of the incoming light through thefirst optical output of the first two-by-two optical splitter and intothe third optical waveguide. The method also includes directing some ofthe polarization-rotated second portion of the incoming light throughthe second optical output of the first two-by-two optical splitter andinto the fourth optical waveguide. The method also includes operating asecond phase shifter interfaced with either the third optical waveguideor the fourth optical waveguide to apply a controlled amount of shift toa phase of light traveling through either the third optical waveguide orthe fourth optical waveguide to which the phase shifter is interfaced.The method also includes directing said some of the first portion of theincoming light and said some of the polarization-rotated second portionof the incoming light from the third optical waveguide into a firstoptical input of a second two-by-two splitter. The method also includesdirecting said some of the first portion of the incoming light and saidsome of the polarization-rotated second portion of the incoming lightfrom the fourth optical waveguide into a second optical input of thesecond two-by-two splitter. The method also includes directing part ofsaid some of the first portion of the incoming light and part of saidsome of the polarization-rotated second portion of the incoming lightthrough an optical output of the second two-by-two splitter and into afifth optical waveguide.

In an example embodiment, an electro-optic transmitter is disclosed. Theelectro-optic transmitter includes a plurality of optical input ports.The electro-optic transmitter also includes a plurality of polarizationcontrollers. Each of the plurality of polarization controllers has anoptical input optically connected to a respective one of the pluralityof optical input ports. Each of the plurality of polarizationcontrollers is configured to convert two polarizations of incoming lightas received through the respective one of the plurality of optical inputports into light having a single polarization, and output the lighthaving the single polarization through an output optical waveguide ofthe polarization controller. The electro-optic transmitter also includesan optical multiplexer that has a plurality of optical inputsrespectively optically connected to the output optical waveguides of theplurality of polarization controllers. The optical multiplexer has aplurality of optical outputs. The electro-optic transmitter alsoincludes a plurality of optical waveguides. Each of the plurality ofoptical waveguides has a first end and second end. The first end of eachof the plurality of optical waveguides is respectively opticallyconnected to the plurality of optical outputs of the opticalmultiplexer. The electro-optic transmitter also includes a plurality ofring resonator modulators positioned along each of the plurality ofoptical waveguides. The electro-optic transmitter also includes aplurality of optical output ports. The second end of each of theplurality of optical waveguides is respectively optically connected tothe plurality of optical output ports.

In an example embodiment, a method is disclosed for operating anelectro-optic transmitter. The method includes receiving incoming lightthrough a plurality of optical input ports. The method also includesoperating a plurality of polarization controllers. Each of the pluralityof polarization controllers has an optical input respectively opticallyconnected to the plurality of optical input ports. Each of the pluralityof polarization controllers is operated to convert light having twopolarizations as received through a corresponding one of the pluralityof optical input ports into light having a single polarization. Each ofthe plurality of polarization controllers is operated to direct thelight having the single polarization through an output optical waveguideof the polarization controller. The method also includes operating anoptical multiplexer that has a plurality of optical inputs respectivelyoptically connected to the output optical waveguides of the plurality ofpolarization controllers. The optical multiplexer has a plurality ofoptical outputs. The optical multiplexer is operated to direct a portionof light received at each of the plurality of optical inputs of theoptical multiplexer to each of the plurality of optical outputs of theoptical multiplexer. The method also includes directing light from eachof the plurality of optical outputs of the optical multiplexer throughrespective ones of a plurality of optical waveguides. Each of theplurality of optical waveguides has a first end and second end. Thefirst end of each of the plurality of optical waveguides is respectivelyoptically connected to the plurality of optical outputs of the opticalmultiplexer. The second end of each of the plurality of opticalwaveguides is respectively optically connected to a plurality of opticaloutput ports. The method also includes operating a plurality of ringresonator modulators positioned along a given one of the plurality ofoptical waveguides to modulate light with the given one of the pluralityof optical waveguides in accordance with a digital bit pattern.

In an example embodiment, an electro-optic transmitter is disclosed. Theelectro-optic transmitter includes a first polarization splitter androtator having an optical input optically connected to receive incominglight. The first polarization splitter and rotator has a first opticaloutput and a second optical output. The first polarization splitter androtator is configured to direct a first portion of the incoming lighthaving a first polarization through the first optical output. The firstpolarization splitter and rotator is configured to rotate a polarizationof a second portion of the incoming light from a second polarization tothe first polarization so that the second portion of the incoming lightis a polarization-rotated second portion of the incoming light. Thefirst polarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic transmitter also includes afirst optical waveguide that has a first end and a second end. The firstend of the first optical waveguide is optically connected to the firstoptical output of the first polarization splitter and rotator. Theelectro-optic transmitter also includes a second optical waveguide thathas a first end and a second end. The first end of the second opticalwaveguide is optically connected to the second optical output of thefirst polarization splitter and rotator. The electro-optic transmitteralso includes a second polarization splitter and rotator that has afirst reverse-connected optical output optically connected to the secondend of the first optical waveguide. The second polarization splitter androtator has a second reverse-connected optical output opticallyconnected to the second end of the second optical waveguide. The secondpolarization splitter and rotator has a reverse-connected optical input.The second polarization splitter and rotator is connected in a reversedmanner with respect to light propagation through the second polarizationsplitter and rotator. The second polarization splitter and rotator isconnected to direct light having the first polarization as received fromthe first optical waveguide through the first reverse-connected opticaloutput to the reverse-connected optical input of the second polarizationsplitter and rotator. The second polarization splitter and rotator isconfigured to derotate a polarization of the polarization-rotated secondportion of the incoming light as received from the second opticalwaveguide through the second reverse-connected optical output from thefirst polarization back to the second polarization, so as to produce apolarization-derotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-derotated second portion of the incoming light to thereverse-connected optical input of the second polarization splitter androtator. The electro-optic transmitter also includes a plurality of ringresonator modulator pairs positioned along the first optical waveguideand the second optical waveguide. Each ring resonator modulator pair ofthe plurality of ring resonator modulator pairs includes one ringresonator modulator positioned within an evanescent optical couplingdistance of the first optical waveguide and one ring resonator modulatorpositioned within an evanescent optical coupling distance of the secondoptical waveguide.

In an example embodiment, a method is disclosed for optical modulation.The method includes receiving incoming light through an optical inputport. A first portion of the incoming light has a first polarization,and a second portion of the incoming light has a second polarization.The method also includes splitting the first portion of the incominglight from the second portion of the incoming light. The method alsoincludes directing the first portion of the incoming light through afirst optical waveguide. The method also includes rotating the secondpolarization of the second portion of the incoming light to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light through a second optical waveguide. The method alsoincludes operating a plurality of ring resonator modulator pairspositioned along the first optical waveguide and the second opticalwaveguide. Each ring resonator modulator pair of the plurality of ringresonator modulator pairs includes one ring resonator modulatorpositioned within an evanescent optical coupling distance of the firstoptical waveguide and one ring resonator modulator positioned within anevanescent optical coupling distance of the second optical waveguide.Each of the plurality of ring resonator modulator pairs is configured tooperate at a specified resonant wavelength to modulate a same bitpattern onto light traveling through the first and second opticalwaveguides to create a first portion of modulated light having the firstpolarization within the first optical waveguide and to create a secondportion of modulated light having the first polarization within thesecond optical waveguide. The method also includes rotating apolarization of the second portion of modulated light within the secondoptical waveguide back from the first polarization to the secondpolarization. The method also includes directing both the first portionof modulated light having the first polarization and the second portionof modulated light having the second polarization through a same opticaloutput port.

In an example embodiment, an electro-optic transmitter is disclosed. Theelectro-optic transmitter includes a first polarization splitter androtator that has an optical input optically connected to receiveincoming light. The first polarization splitter and rotator has a firstoptical output and a second optical output. The first polarizationsplitter and rotator is configured to direct a first portion of theincoming light that has a first polarization through the first opticaloutput. The first polarization splitter and rotator is configured torotate a polarization of a second portion of the incoming light from asecond polarization to the first polarization so that the second portionof the incoming light is a polarization-rotated second portion of theincoming light. The first polarization splitter and rotator isconfigured to direct the polarization-rotated second portion of theincoming light through the second optical output. The electro-optictransmitter also includes a first optical waveguide that has a first endand second end. The first end of the first optical waveguide isoptically connected to the first optical output of the firstpolarization splitter and rotator. The electro-optic transmitter alsoincludes a second optical waveguide that has a first end and second end.The first end of the second optical waveguide is optically connected tothe second optical output of the first polarization splitter androtator. The electro-optic transmitter also includes a two-by-twooptical splitter that has a first optical input optically connected tothe second end of the first optical waveguide. The two-by-two opticalsplitter has a second optical input optically connected to the secondend of the second optical waveguide. The two-by-two optical splitter hasa first optical output and a second optical output. The two-by-twooptical splitter is configured to output some of the first portion ofthe incoming light and some of the polarization-rotated second portionof the incoming light through each of the first optical output and thesecond optical output of the two-by-two optical splitter. Theelectro-optic transmitter also includes a third optical waveguide thathas a first end and second end. The first end of the third opticalwaveguide is optically connected to the first optical output of thetwo-by-two optical splitter. The electro-optic transmitter also includesa fourth optical waveguide that has a first end and second end. Thefirst end of the fourth optical waveguide is optically connected to thesecond optical output of the two-by-two optical splitter. Theelectro-optic transmitter also includes a second polarization splitterand rotator that has a first reverse-connected optical output opticallyconnected to the second end of the third optical waveguide. The secondpolarization splitter and rotator has a second reverse-connected opticaloutput optically connected to the second end of the fourth opticalwaveguide. The second polarization splitter and rotator has areverse-connected optical input. The second polarization splitter androtator is connected in a reversed manner with respect to lightpropagation through the second polarization splitter and rotator. Thesecond polarization splitter and rotator is connected to direct lightreceived through the first reverse-connected optical output of thesecond polarization splitter and rotator to the reverse-connectedoptical input of the second polarization splitter and rotator. Thesecond polarization splitter and rotator is configured to derotate apolarization of light received through the second reverse-connectedoptical output of the second polarization splitter and rotator from thefirst polarization the second polarization so as to producepolarization-derotated light. The polarization splitter and rotator isconfigured to direct the polarization-derotated light to thereverse-connected optical input of the second polarization splitter androtator. The electro-optic transmitter also includes a plurality of ringresonator modulator pairs positioned along the third optical waveguideand the fourth optical waveguide. Each ring resonator modulator pair ofthe plurality of ring resonator modulator pairs includes one ringresonator modulator positioned within an evanescent optical couplingdistance of the third optical waveguide and one ring resonator modulatorpositioned within an evanescent optical coupling distance of the fourthoptical waveguide.

In an example embodiment, a method is disclosed for optical modulation.The method includes receiving incoming light through an optical inputport. A first portion of the incoming light has a first polarization,and a second portion of the incoming light has a second polarization.The method also includes splitting the first portion of the incominglight from the second portion of the incoming light. The method alsoincludes directing the first portion of the incoming light through afirst optical waveguide and into a first optical input of a two-by-twosplitter. The method also includes rotating the second polarization ofthe second portion of the incoming light to the first polarization sothat the second portion of the incoming light is a polarization-rotatedsecond portion of the incoming light. The method also includes directingthe polarization-rotated second portion of the incoming light through asecond optical waveguide and into a second optical input of thetwo-by-two splitter. The method also includes directing some of thefirst portion of the incoming light through a first optical output ofthe two-by-two optical splitter and into a third optical waveguide. Themethod also includes directing some of the first portion of the incominglight through a second optical output of the two-by-two optical splitterand into a fourth optical waveguide. The method also includes directingsome of the polarization-rotated second portion of the incoming lightthrough the first optical output of the two-by-two optical splitter andinto the third optical waveguide. The method also includes directingsome of the polarization-rotated second portion of the incoming lightthrough the second optical output of the two-by-two optical splitter andinto the fourth optical waveguide. The method also includes operating aplurality of ring resonator modulator pairs positioned along the thirdoptical waveguide and the fourth optical waveguide. Each ring resonatormodulator pair of the plurality of ring resonator modulator pairsincludes one ring resonator modulator positioned within an evanescentoptical coupling distance of the third optical waveguide and one ringresonator modulator positioned within an evanescent optical couplingdistance of the fourth optical waveguide. Each of the plurality of ringresonator modulator pairs is configured to operate at a specifiedresonant wavelength to modulate a same bit pattern onto light travelingthrough the third optical waveguide and the fourth optical waveguide.The method also includes rotating a polarization of modulated lightwithin either the third optical waveguide or the fourth opticalwaveguide from the first polarization to the second polarization. Themethod also includes directing both modulated light that has the firstpolarization and modulated light that has the second polarizationthrough a same optical output port.

In an example embodiment, an electro-optic combiner is disclosed. Theelectro-optic combiner includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light that has afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic combiner also includes a firstoptical waveguide that has a first end and a second end. The first endof the first optical waveguide is optically connected to the firstoptical output of the polarization splitter and rotator. Theelectro-optic combiner also includes a second optical waveguide that hasa first end and a second end. The first end of the second opticalwaveguide is optically connected to the second optical output of thepolarization splitter and rotator. The electro-optic combiner alsoincludes a plurality of ring resonators disposed between a combinersection of the first optical waveguide and a combiner section of thesecond optical waveguide. Each of the plurality of ring resonators ispositioned within an evanescent optically coupling distance of both thefirst optical waveguide and the second optical waveguide. A lightpropagation direction through the combiner section of the first opticalwaveguide is opposite of a light propagation direction through thecombiner section of the second optical waveguide. Each of the pluralityof ring resonators is configured to operate at a respective resonantwavelength, such that light having a wavelength substantially equal tothe respective resonant wavelength of a given one of the plurality ofring resonators optically couples light from the combiner section of thefirst optical waveguide into the given one of the plurality of ringresonators, and from the given one of the plurality of ring resonatorsinto the second optical waveguide.

In an example embodiment, a method is disclosed for combination ofoptical signals. The method includes receiving incoming light through anoptical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightthrough a first optical waveguide. The method also includes rotating thesecond polarization of the second portion of the incoming light to thefirst polarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light through a second optical waveguide. The method alsoincludes operating a plurality of ring resonators disposed between thefirst optical waveguide and the second optical waveguide. Each of theplurality of ring resonators is operated to evanescently in-couple lightfrom the first optical waveguide and out-couple light into the secondoptical waveguide. Each of the plurality of ring resonators isconfigured to operate at a respective resonant wavelength, such thatlight having a wavelength substantially equal to the respective resonantwavelength of a given one of the plurality of ring resonators opticallycouples light from the first optical waveguide into the given one of theplurality of ring resonators, and from the given one of the plurality ofring resonators into the second optical waveguide.

In an example embodiment, an electro-optic combiner is disclosed. Theelectro-optic combiner includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light that has afirst polarization through the first optical output. The polarizationsplitter and rotator is configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic combiner also includes a firstoptical waveguide optically connected to the first optical output of thepolarization splitter and rotator. The electro-optic combiner alsoincludes a first plurality of ring resonators positioned along the firstoptical waveguide, such that the phase shifter is positioned alongsidethe first optical waveguide before the first plurality of ringresonators relative to a direction of light propagation through thefirst optical waveguide. Each of the first plurality of ring resonatorsis positioned within an evanescent optical coupling distance of thefirst optical waveguide. The electro-optic combiner also includes asecond optical waveguide optically connected to the second opticaloutput of the polarization splitter and rotator. The electro-opticcombiner also includes a second plurality of ring resonators positionedalong the second optical waveguide and within an evanescent opticalcoupling distance of the second optical waveguide. Each of the secondplurality of ring resonators is positioned to optically in-couple lightfrom a respective one of the first plurality of ring resonators andoptically out-couple light into the second optical waveguide.

In an example embodiment, a method is disclosed for combination ofoptical signals. The method includes receiving incoming light through anoptical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightthrough a first optical waveguide. The method also includes rotating thesecond polarization of the second portion of the incoming light to thefirst polarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light through a second optical waveguide. The method alsoincludes operating a first plurality of ring resonators disposed betweenthe first optical waveguide and the second optical waveguide. Each ofthe first plurality of ring resonators is operated to evanescentlyin-couple light from the first optical waveguide. The method alsoincludes operating a second plurality of ring resonators disposedbetween the first optical waveguide and the second optical waveguide.Each of the second plurality of ring resonators is operated toevanescently in-couple light from a respective one of the firstplurality of ring resonators. Each of the second plurality of ringresonators is operated to evanescently out-couple light to the secondoptical waveguide. Each optically coupled pair of ring resonators withinthe first and second pluralities of ring resonators is operated at asubstantially same resonant wavelength. Each optically coupled pair ofring resonators within the first and second pluralities of ringresonators has opposite light propagation directions.

In an example embodiment, an electro-optic combiner is disclosed. Theelectro-optic combiner includes a polarization splitter and rotator thathas an optical input optically connected to receive incoming light. Thepolarization splitter and rotator has a first optical output and asecond optical output. The polarization splitter and rotator isconfigured to direct a first portion of the incoming light that has afirst polarization through the first optical output. The polarizationsplitter and rotator configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. Thepolarization splitter and rotator is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output. The electro-optic combiner also includes a firstoptical waveguide optically connected to the first optical output of thepolarization splitter and rotator. The electro-optic combiner alsoincludes a first plurality of ring resonators positioned along the firstoptical waveguide such that the phase shifter is positioned alongsidethe first optical waveguide before the first plurality of ringresonators relative to a direction of light propagation through thefirst optical waveguide. Each of the first plurality of ring resonatorsis positioned within an evanescent optical coupling distance of thefirst optical waveguide. The electro-optic combiner also includes asecond optical waveguide optically connected to the second opticaloutput of the polarization splitter and rotator. The electro-opticcombiner also includes a second plurality of ring resonators positionedalong the second optical waveguide and within an evanescent opticalcoupling distance of the second optical waveguide. The electro-opticcombiner also includes a plurality of intermediate optical waveguides.Each of the plurality of intermediate optical waveguides is positionedbetween a corresponding one of the first plurality of ring resonatorsand a corresponding one of the second plurality of ring resonators, suchthat light optically couples from the first optical waveguide to thecorresponding one of the first plurality of ring resonators, and fromthe corresponding one of the first plurality of ring resonators to saidintermediate optical waveguide, and from said intermediate opticalwaveguide to the corresponding one of the second plurality of ringresonators, and from the corresponding one of the second plurality ofring resonators to the second optical waveguide. The electro-opticcombiner also includes a plurality of photodetectors respectivelyoptically connected to the plurality of intermediate optical waveguides,such that some of the light that optically couples into a given one ofthe plurality of intermediate optical waveguides from the correspondingone of the first plurality of ring resonators is conveyed into one ofthe plurality of photodetectors that is optically connected to the givenone of the plurality of intermediate optical waveguides.

In an example embodiment, a method is disclosed for combination ofoptical signals. The method includes receiving incoming light through anoptical input port. A first portion of the incoming light has a firstpolarization, and a second portion of the incoming light has a secondpolarization. The method also includes splitting the first portion ofthe incoming light from the second portion of the incoming light. Themethod also includes directing the first portion of the incoming lightthrough a first optical waveguide. The method also includes rotating thesecond polarization of the second portion of the incoming light to thefirst polarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes directing the polarization-rotated second portion of theincoming light through a second optical waveguide. The method alsoincludes operating a first plurality of ring resonators disposed betweenthe first optical waveguide and the second optical waveguide. Each ofthe first plurality of ring resonators is operated to evanescentlyin-couple light from the first optical waveguide and evanescentlyout-couple light to a corresponding one of a plurality of intermediateoptical waveguides. The method also includes operating a secondplurality of ring resonators disposed between the first opticalwaveguide and the second optical waveguide. Each of the second pluralityof ring resonators is operated to evanescently in-couple light from acorresponding one of the plurality of intermediate optical waveguides.Each of the second plurality of ring resonators is operated toevanescently out-couple light to the second optical waveguide. Each pairof ring resonators within the first and second pluralities of ringresonators that are optically coupled to a same one of the plurality ofintermediate optical waveguides is operated at a substantially sameresonant wavelength. Each pair of ring resonators within the first andsecond pluralities of ring resonators that are optically coupled to thesame one of the plurality of intermediate optical waveguides hasopposite light propagation directions.

Other aspects and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example configuration of an electro-optic receiver, inaccordance with some embodiments.

FIG. 1B shows an example configuration of an electro-optic receiverimplemented within a PIC, in accordance with some embodiments.

FIG. 1C shows an example configuration of a PSR, in accordance with someembodiments.

FIG. 1D shows a vertical cross-section view through the example PSR,referenced as View A-A in FIG. 1C, in accordance with some embodiments.

FIG. 1E shows an example configuration of a PSR, in accordance with someembodiments.

FIG. 1F shows a vertical cross-section view through the example PSR,referenced as View A-A in FIG. 1E, in accordance with some embodiments.

FIG. 1G shows a vertical cross-section view through the example PSR,referenced as View B-B in FIG. 1E, in accordance with some embodiments.

FIG. 2A shows an example of a WDM optical receiver that includesmultiple ring resonator photodetectors positioned along an opticalwaveguide that is configured to extend in a continuous, loop-likeconfiguration, in accordance with some embodiments.

FIG. 2B shows a WDM optical receiver that is modified version of the WDMoptical receiver of FIG. 2A, in accordance with some embodiments.

FIG. 2C shows a WDM optical receiver that is modified version of the WDMoptical receiver of FIG. 2B, in accordance with some embodiments.

FIG. 3 shows an example configuration of an electro-optic receiverimplemented within a PIC, in accordance with some embodiments.

FIG. 4 shows a diagram of an example linear photodetector, in accordancewith some embodiments.

FIG. 5 shows a flowchart of a method for operating a photonic circuit,in accordance with some embodiments.

FIG. 6 shows an example configuration of an electro-optic receiverimplemented within a PIC, in accordance with some embodiments.

FIG. 7 shows a flowchart of a method for operating a photonic circuit,in accordance with some embodiments.

FIG. 8 shows an example configuration of an electro-optic receiverimplemented within a PIC, in accordance with some embodiments.

FIG. 9 shows a flowchart of a method for operating a photonic integratedcircuit, in accordance with some embodiments.

FIG. 10A shows an example configuration of an optical input polarizationmanagement device implemented within a PIC, in accordance with someembodiments.

FIG. 10B shows the optical input polarization management device of FIG.10A, with an example implementation of the polarization controller, inaccordance with some embodiments.

FIG. 10C shows an example implementation of the optical inputpolarization management device in which the first phase shifter isimplemented as a first plurality of ring resonator phase shifters andthe second phase shifter is implemented as a second plurality of ringresonator phase shifters, in accordance with some embodiments.

FIG. 11 shows a flowchart of a method for optical input polarizationmanagement, in accordance with some embodiments.

FIG. 12 shows an example configuration of an electro-optic transmitterimplemented within a PIC, in accordance with some embodiments.

FIG. 13 shows a flowchart of a method for operating an electro-optictransmitter, in accordance with some embodiments.

FIG. 14 shows an example configuration of an electro-optic transmitterimplemented within a PIC, in accordance with some embodiments.

FIG. 15 shows a flowchart of a method for optical modulation, inaccordance with some embodiments.

FIG. 16 shows an example configuration of an electro-optic transmitterimplemented within a PIC, in accordance with some embodiments.

FIG. 17 shows a flowchart of a method for optical modulation, inaccordance with some embodiments.

FIG. 18 shows an example configuration of an electro-optic combinerimplemented within a PIC, in accordance with some embodiments.

FIG. 19 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments.

FIG. 20 shows an example configuration of an electro-optic combinerimplemented within a PIC, in accordance with some embodiments.

FIG. 21 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments.

FIG. 22 shows an example configuration of an electro-optic combinerimplemented within a PIC, in accordance with some embodiments.

FIG. 23 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments.

FIG. 24A shows a diagram of an electro-optic receiver that is configuredto tolerate polarization-dependent timing-skew, in accordance with someembodiments.

FIG. 24B shows a modification of the electro-optic receiver of FIG. 24A,in accordance with some embodiments.

FIG. 24C shows a modification of the electro-optic receiver of FIG. 24B,in accordance with some embodiments.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide an understanding of the disclosed embodiments. It willbe apparent, however, to one skilled in the art that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the disclosedembodiments.

Optical data communication systems operate by modulating laser light toencode digital data patterns within the electrical domain as modulatedlight signals within the optical domain. The modulated light signals aretransmitted through optical fibers to an electro-optic receiver wherethe modulated light signals are detected and decoded to obtain theoriginal encoded digital data patterns back in the electrical domain. Inmany optical data communication systems, a polarization state of thelight within the optical fiber is not controlled, and may be perturbedby small movements of the optical fiber and/or changes in ambienttemperature while the system is operating. In these systems, theelectro-optic receiver has to handle incoming light signals that have anarbitrary polarization that varies over time.

Electro-optic receiver systems are often built into photonic integratedcircuits (PIC's), enabling compact and high-performance detection ofmodulated light signals received as input from optical fibers. Opticalcoupling of light from an optical fiber into a PIC requires an opticalcoupling configuration that can accept input light from eitherpolarization (transverse electric (TE) or transverse magnetic (TM)) ofan optical fiber and output it to one or more optical waveguides on thePIC, and often into a preferred polarization state. In some embodimentsdisclosed herein, an optical coupling configuration is provided in whichincoming light is received through either a dual-polarization verticalgrating coupler or an edge coupler and is conveyed into a PICpolarization splitter, which splits the incoming light from the twoinput optical fiber polarizations (TE and TM) and outputs the incominglight of a first polarization and a second polarization into twoseparate optical waveguides on the PIC, respectively. Also, in someembodiments, either the first polarization or the second polarization isrotated to the other polarization in route to the two separate opticalwaveguides on the PIC, such that light having the same polarization isconveyed into each of the two separate optical waveguides on the PIC. Insome implementations, a significant advantage is gained by using opticaldevices that can efficiently detect optical signals that are split inthis way based on polarization. Also, in some implementations, there arefurther advantages obtained by using one photodiode (such as in aphotodetector) for both polarization mode components of the incominglight rather than duplicating the number of photodiodes to provide forseparate detection of the two polarization mode components of theincoming light, where such further advantages include decreasedcomplexity of the optical circuitry, reduced detector capacitance perchannel, and reduced dark current, which results in increasedphotodiode/photodetector sensitivity.

Various embodiments are disclosed herein for an electro-optic receiver.The electro-optic receiver enables the detection of optical signals ofarbitrary input polarization with a single photodetector or set ofphotodetectors. The electro-optic receiver includes an input opticalfiber carrying incoming light (modulated light) with arbitrary(uncontrolled) polarization that conveys a signal that is decodable to adigital data pattern. The electro-optic receiver also includes anoptical coupling device that transfers the incoming light from the inputfiber to a PIC. The electro-optic receiver also includes a polarizationbeam splitter that receives an arbitrary input polarization state andsplits it into two separate optical waveguides of the PIC, where eachoptical waveguide contains one of the orthogonal components of the inputpolarization state of the incoming light, possibly converted into adifferent polarization state. In some embodiments of the electro-opticreceiver, the functionality of the polarization beam splitter iscombined with the optical coupling device in the form of adual-polarization grating coupler. In some embodiments of theelectro-optic receiver, the two optical waveguides of the PIC support asingle polarization mode that is well isolated in propagation constantfrom other spatial modes, including the other polarization state, makingit operationally single-mode, single-polarization. The electro-opticreceiver also includes an optical routing system in which the twooptical waveguides of the PIC are routed to the same photodetector orset of photodetectors. The electro-optic receiver also includes atiming-skew management system that corrects for degradation in thesignal arising from time delay mismatch between the two opticalwaveguides of the PIC and the photodetector or set of photodetectors.

The electro-optic receiver disclosed herein is particularly useful inapplications where electro-optic receivers implemented within a PICdetect light from an input optical fiber in which the polarization isnot controlled. The electro-optic receiver is especially advantageous inapplications that encode multiple data channels on the input opticalfiber on different wavelengths, for example using wavelength divisionmultiplexing (WDM), and in cases where polarization-diversity WDMelectro-optic receiver architectures have a timing-skew. Thistiming-skew may vary from one channel to the next, typically in a knownway. Some existing electro-optic receivers that are capable of handlinguncontrolled input polarization require two separate electro-opticreceivers, one for each orthogonal polarization state, together withextensive digital signal processing to combine the signals. It should beappreciated that the electro-optic receiver embodiments disclosed hereinprovide a single, compact, and power-efficient electro-optic receiver tocombine and detect signals from the two polarization states of anywavelength channel.

Some embodiments of the electro-optic receiver disclosed herein areparticularly useful in situations in which the polarization states ofthe incoming light of the different wavelength channels are roughly thesame from one wavelength channel to the next, and where the polarizationstates of the incoming light of the different wavelength channels areunknown. Some embodiments of the electro-optic receiver disclosed hereinare particularly useful in situations in which the polarization state ofthe incoming light of a given wavelength channel is unknown, and wherethe polarization state of the incoming light of the given wavelengthchannel varies in a slow and controlled manner (such as monotonically)as compared to other wavelength channels of the incoming light. Forexample, these situations may occur when the WDM signal wavelengthchannels all originate at the same source, traverse the same opticalfiber, and are received together, as is the case in a WDM point-to-pointlink. Some embodiments of the electro-optic receiver disclosed hereinare useful in a more generalized situation in which each wavelengthchannel of the incoming light has a completely different unknownpolarization, and where the polarization states of the differentwavelength channels of the incoming light are uncorrelated. For example,these situations may occur when different WDM channels originate indifferent locations and may not share the same optical fiber(s) over theentire propagation path from the respective sources of the incominglight to the electro-optic receiver. The electro-optic receiverembodiments disclosed herein are configured to handle incoming lightsignals whose polarization state is unknown and is either static in timeor dynamically changing in time. Various embodiments of theelectro-optic receiver disclosed herein provide for reception ofincoming light signals having unknown polarization states, even if thepolarization states changes at high speeds, e.g., into the gigaHertz(GHz) regime, but more typically in the kiloHertz (kHz) (or millisecond)regime.

FIG. 1A shows an example configuration of an electro-optic receiver 100,in accordance with some embodiments. The electro-optic receiver 100includes a PIC 101 into which incoming light is received from an opticalfiber 103. The PIC includes an optical coupler 105 configured to receivethe incoming light from the optical fiber 103 and direct the incominglight into an optical waveguide 107 within the PIC 101. The incominglight conveys one or more optical signals. For example, the incominglight is modulated light that optically conveys a digital bit pattern.The incoming light also does not have polarization control. Therefore,the polarization of the incoming light received from the optical fiber103 is unknown when it enters into the optical coupler 105. In theexample electro-optic receiver 100, the optical coupler 105 is avertical grating coupler. In some embodiments, the optical coupler 105is configured as a dual-polarization grating coupler (either as an edgegrating coupler or as a vertical grating coupler) that splits the twopolarizations (TE and TM) of incoming light. The dual-polarizationgrating coupler is configured to direct a first portion of the incominglight having a first polarization (either TE or TM) into a first end107A of the optical waveguide 107. The dual-polarization grating coupleris also configured to rotate a polarization of a second portion of theincoming light from a second polarization that is opposite of the firstpolarization (e.g., the second polarization is TM, if the firstpolarization is TE, and vice-versa) to the first polarization so as toprovide a polarization-rotated second portion of the incoming light thathas the first polarization. The dual-polarization grating coupler isalso configured to direct the polarization-rotated second portion of theincoming light into a second end 107B of the optical waveguide 107. Theoptical waveguide 107 is configured to extend in a continuous, loop-likeconfiguration from the first end 107A to the second end 107B. In thismanner, the first portion of the incoming light having the firstpolarization travels in a first light propagation direction through theoptical waveguide 107 from the first end 107A toward the second end107B, and the polarization-rotated second portion of the incoming light(also having the first polarization) travels in a second lightpropagation direction through the optical waveguide 107 from the secondend 107B toward the first end 107A. Therefore, the first portion of theincoming light and the polarization-rotated second portion of theincoming light travel in opposite light propagation directions throughthe optical waveguide 107. In some embodiments, each of the first end107A and the second end 107B of the optical waveguide 107 is configuredas a respective tapered region of the optical waveguide 107 tofacilitate optical coupling of light from the dual-polarization gratingcoupler into the optical waveguide 107.

The electro-optic receiver 100 also includes multiple ring resonatorphotodetectors 109-1 to 109-n, where n is an integer value greaterthan 1. The ring resonator photodetectors 109-1 to 109-n are positionedalongside the optical waveguide 107 and within an evanescent opticalcoupling distance of the optical waveguide 107. Each of the ringresonator photodetectors 109-1 to 109-n is configured to operate at arespective resonant wavelength. In some embodiments, the respectiveresonant wavelength at which any one of the ring resonatorphotodetectors 109-1 to 109-n operates is a narrow wavelength range. Forease of description, any ring resonator disclosed herein is described asoperating a respective resonant wavelength, with the understanding thatthe respective resonant wavelength is actually a narrow wavelength rangethat is distinguishable from other different resonant wavelength ranges.In this manner, each of the ring resonator photodetectors 109-1 to 109-nis configured to detect light at the respective resonant wavelength(within the narrow wavelength range about the respective resonantwavelength). In some embodiments, each of the ring resonatorphotodetectors 109-1 to 109-n is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 50 micrometers. Insome embodiments, each of the ring resonator photodetectors 109-1 to109-n is configured to have an annular-shape or disc-shape with an outerdiameter of less than about 10 micrometers.

Each of the ring resonator photodetectors 109-1 to 109-n is configuredto operate at a respective resonant wavelength, such that the firstportion of the incoming light having a wavelength substantially equal tothe respective resonant wavelength of a given one of the ring resonatorphotodetectors 109-x optically couples into the given one of the ringresonator photodetectors 109-x in a first propagation direction, andsuch that the polarization-rotated second portion of the incoming lighthaving a wavelength substantially equal to the respective resonantwavelength of the given one of the ring resonator photodetectors 109-xalso optically couples into the given one of the ring resonatorphotodetectors 109-x in a second propagation direction opposite thefirst propagation direction. For example, a particular wavelength of thefirst portion of the incoming light traveling from the first end 107A ofthe optical waveguide 107 toward the second end 107B of the opticalwaveguide 107 will optically couple into one or more of the ringresonator photodetectors 109-1 to 109-n operating at a resonantwavelength substantially equal to the particular wavelength, such thatthe particular wavelength of the first portion of the incoming lightpropagates in a counter-clockwise direction within the one or more ringresonator photodetectors 109-1 to 109-n into which it optically couples.Conversely, a particular wavelength of the polarization-rotated secondportion of the incoming light traveling from the second end 107B of theoptical waveguide 107 toward the first end 107A of the optical waveguide107 will optically couple into one or more of the ring resonatorphotodetectors 109-1 to 109-n operating at a resonant wavelengthsubstantially equal to the particular wavelength, such that theparticular wavelength of the polarization-rotated portion of theincoming light propagates in a clockwise direction within the one ormore ring resonator photodetectors 109-1 to 109-n into which itoptically couples.

Because the first portion of the incoming light and the correspondingpolarization-rotated second portion of the incoming light may not arriveat a given one of the ring resonator photodetectors 109-x at the sametime, the electro-optic receiver 100 also includes a timing-skewmanagement system 111 that is configured to identify and compensate forthe temporal differences in arrival time of the first portion of theincoming light and the corresponding polarization-rotated second portionof the incoming light at a given one of the ring resonatorphotodetectors 109-x in order to provide for recovery of the opticalsignal as conveyed within the incoming light. The difference in arrivaltime of the first portion of the incoming light and the correspondingpolarization-rotated second portion of the incoming light at a given oneof the ring resonator photodetectors 109-x can be caused by differencesin optical path length through the optical waveguide 107 to the givenone of the ring resonator photodetectors 109-x and/or by delay inoutputting the polarization-rotated second portion of the incoming lightrelative to the first portion of the incoming light from the opticalcoupler 105 (from the dual-polarization grating coupler).

FIG. 1B shows an example configuration of an electro-optic receiver 150implemented within a PIC 151, in accordance with some embodiments. Theelectro-optic receiver 150 receives the incoming optical signal from anoptical fiber/waveguide 152 through an optical coupler 153, as indicatedby arrow 154. In some embodiments, the optical coupler 153 isimplemented as an edge coupler. However, in other embodiments, theoptical coupler 153 is implemented as a vertical grating coupler, or asanother type of optical coupling device that provides for opticalcoupling of the PIC 151 to the optical fiber/waveguide 152. The incomingoptical signal is conveyed from the optical coupler 153 through anoptical waveguide 155 to an optical input of a polarization splitter androtator (PSR) 156 of the electro-optic receiver 150. In this manner, thePSR 156 has an optical input 156A optically connected to receiveincoming light. In some embodiments, the optical input 156A of the PSR156 is directly optically coupled to the optical coupler 153, such thatthe optical waveguide 155 is not required. The PSR 156 has a firstoptical output 156B and a second optical output 156C. The PSR 156 isconfigured to direct a first portion of the incoming light having afirst polarization through the first optical output 156B. The PSR 156 isconfigured to rotate a polarization of a second portion of the incominglight from a second polarization to the first polarization. In thismanner, the PSR 156 turns the second portion of the incoming light intoa polarization-rotated second portion of the incoming light. The PSR 156is configured to direct the polarization-rotated second portion of theincoming light through the second optical output 156C.

The electro-optic receiver 150 includes an optical waveguide 157 formedwithin the PIC 151. The optical waveguide 157 has a first end 157Aoptically to the first optical output 156B of the PSR 156. The opticalwaveguide 157 also has a second end 157B optically connected to thesecond optical output 156C of the PSR 156. In this manner, the firstportion of the incoming light travels from the first optical output 156Aof the PSR 156 through the optical waveguide 157 in a first direction,as indicated by arrows 158. Also, the polarization-rotated secondportion of the incoming light travels from the second optical output156C of the PSR 156 through the optical waveguide 157 in a seconddirection, as indicated by arrows 159, that is opposite the firstdirection. The optical waveguide 157 is formed of a material throughwhich light can be in-coupled, out-coupled, and guided. The opticalwaveguide 157 is formed within a surrounding material that has anoptical index of refraction sufficiently different from that of theoptical waveguide 157 to enable guiding of light within the opticalwaveguide 157.

The electro-optic receiver 150 also includes a plurality of ringresonator photodetectors 161-1 to 161-6 positioned alongside the opticalwaveguide 157 and within an evanescent optical coupling distance of theoptical waveguide 157. It should be understood that the number of thering resonator photodetectors 161-1 to 161-6 is provided by way ofexample. In some embodiments, the electro-optic receiver 150 includesless than six ring resonator photodetectors. In some embodiments, theelectro-optic receiver 150 includes more than six ring resonatorphotodetectors. It should be understood that there is no limit on thenumber of the ring resonator photodetectors (e.g., 161-1 to 161-6) thatcan be positioned along the optical waveguide 157, so long as the ringresonator photodetectors and associated signal processing circuitry canbe spatially and electrically accommodated on the chip. In someembodiments, the ring resonator photodetectors 161-1 to 161-6 areimplemented as annular-shaped waveguides having circuitousconfiguration, e.g., circular, oval, race-track, or another arbitrarycircuitous shape. In some embodiments, the ring resonator photodetectors161-1 to 161-6 are implemented as circular discs. The ring resonatorphotodetectors 161-1 to 161-6 are formed of a material through whichlight can be in-coupled, out-coupled, and guided. Each of the ringresonator photodetectors 161-1 to 161-6 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the ring resonator photodetectors 161-1 to 161-6 to enableguiding of light within the ring resonator photodetectors 161-1 to 161-6and around the circuitous path defined by each of the ring resonatorphotodetectors 161-1 to 161-6. In some embodiments, each of the ringresonator photodetectors 161-1 to 161-6 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonatorphotodetectors 161-1 to 161-6 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about micrometers.

Each of the plurality of ring resonator photodetectors 161-1 to 161-6 isconfigured to operate at a respective resonant wavelength λ₁ to λ₆,respectively, such that the first portion of the incoming light having awavelength substantially equal to the respective resonant wavelength ofa given one of the plurality of ring resonator photodetectors 161-1 to161-6 optically couples into the given one of the plurality of ringresonator photodetectors 161-1 to 161-6 in a first propagationdirection, and such that the polarization-rotated second portion of theincoming light having a wavelength substantially equal to the respectiveresonant wavelength of the given one of the plurality of ring resonatorphotodetectors 161-1 to 161-6 optically couples into the given one ofthe plurality of ring resonator photodetectors 161-1 to 161-6 in asecond propagation direction opposite the first propagation direction.For example, if the incoming light has a wavelength substantially equalto the wavelength λ₂, then the first portion of the incoming lighthaving the wavelength λ₂ will optically couple into the ring resonator161-2 and propagate in a counter-clockwise direction within the ringresonator 161-2, and the polarization-rotated second portion of theincoming light having the wavelength λ₂ will also optically couple intothe ring resonator 161-2 and propagate in a clockwise direction withinthe ring resonator 161-2. It should be understood that both the firstportion of the incoming light and the polarization-rotated secondportion of the incoming light have the same polarization state withinthe optical waveguide 157. Therefore, any given one of the ringresonator photodetectors 161-1 to 161-6 operating at a particularresonant wavelength is able to optically in-couple and detect both thefirst portion of the incoming light and the polarization-rotated secondportion of the incoming light having the particular resonant wavelength.

In some embodiments, such as shown in the example electro-optic receiver150, the plurality of ring resonator photodetectors 161-1 to 161-6includes a first set of ring resonator photodetectors 161-1 to 161-3positioned between the first end 157A of the optical waveguide 157 and amidpoint 157C of the optical waveguide 157 located halfway between thefirst end 157A and the second end 157B of the optical waveguide 157.Also, in these embodiments, the plurality of ring resonatorphotodetectors 161-1 to 161-6 includes a second set of ring resonatorphotodetectors 161-4 to 161-6 positioned between the second end 157B ofthe optical waveguide 157 and the midpoint 157C of the optical waveguide157. It should be understood that in some embodiments, the number ofring resonator photodetectors in the first set of ring resonatorphotodetectors is either less than or more than the three ring resonatorphotodetectors 161-1 to 161-3. Also, in some embodiments, the number ofring resonator photodetectors in the second set of ring resonatorphotodetectors is either less than or more than the three ring resonatorphotodetectors 161-4 to 161-6.

In some embodiments, the electro-optic receiver 150 also includes avariable optical attenuator (VOA) 163 that is configured to attenuatethe light propagated through the optical waveguide 157 in a controlledmanner in accordance with an electrical control signal. In someembodiments, the VOA 163 is positioned to optically couple to theoptical waveguide 157 near either the first end 157A or the second end157B of the optical waveguide 157. In the example electro-optic receiver150, the VOA 163 is optically coupled to the optical waveguide 157 nearthe first end 157A of the optical waveguide 157. The VOA 163 operates tolimit the optical reflection/transmission of light from the opticalwaveguide 157 back into the optical fiber/waveguide 152 when theelectro-optic receiver 150 is initially turned on, and before the ringresonator photodetectors 161-1 to 161-6 have been tuned to theirrespective resonant wavelengths λ₁ to λ₆ to in-couple light of thevarious wavelength channels present in the incoming optical signal.During startup of the electro-optic receiver 150, incoming light thatdoes not couple into any of the ring resonator photodetectors 161-1 to161-6 will pass through the optical waveguide 157 and back out throughthe optical fiber/waveguide 152, which could possibly damage the sourcefrom which the incoming light was transmitted. For example, if thesource of the incoming light is a laser source, reverse transmission ofthe incoming laser light back into the laser source could damage thelaser source or degrade its performance. To prevent this from happening,during startup of the electro-optic receiver 150, the VOA 163 operatesto attenuate the light propagating within the optical waveguide 157 to alevel where light returning to the optical fiber/waveguide 152 will notdamage and/or disrupt operation of the source from which the incominglight was transmitted, e.g., the laser source, while also keeping theoptical power within the optical waveguide 157 just high enough to allowthe ring resonator photodetectors 161-1 to 161-6 to be tuned and lockedto their respective resonant wavelengths λ₁ to λ₆ corresponding to thevarious wavelength channels present in the incoming optical signal.Then, after the ring resonator photodetectors 161-1 to 161-6 are tunedand locked to their respective resonant wavelengths λ₁ to λ₆corresponding to the various wavelength channels present in the incomingoptical signal, operation of the VOA 163 is adjusted to reduce or stopattenuation of the light propagating within the optical waveguide 157.

In some embodiments, the VOA 163 is implemented as an optical waveguide(or portion of the optical waveguide 157) having a built-in PN or PINdiode that when forward-biased creates an electrical current within theoptical waveguide of the VOA 163 that increases optical absorptionwithin the optical waveguide of the VOA 163 through free-carrierabsorption. In these embodiments, the optical absorption within theoptical waveguide of the VOA 163 is increased by increasing theforward-bias voltage (and thus by increasing the electrical current)within the optical waveguide of the VOA 163. Also, in these embodiments,the optical absorption within the optical waveguide of the VOA 163 isreduced by reducing the forward bias-voltage (and thus by reducing theelectrical current) within the optical waveguide of the VOA 163. Also,in these embodiments, the optical absorption within the opticalwaveguide of the VOA 163 is stopped by reverse-biasing of the PN or PINdiode within the optical waveguide of the VOA 163. In some embodiments,the PN or PIN diode of the VOA 163 is actually formed within the opticalwaveguide 157. In some embodiments, the VOA 163 includes its own opticalwaveguide separate from the optical waveguide 157, where the opticalwaveguide of the VOA 163 is evanescently optically coupled to a sectionof the optical waveguide 157, with the PN or PIN diode built into theoptical waveguide of the VOA 163.

In some embodiments, the electro-optic receiver 150 also includes atiming-skew management system 165 configured to electronicallycompensate for a temporal difference in photocurrent generation by anygiven one of the plurality of ring resonator photodetectors 161-1 to161-6 caused by a difference in arrival time of the first portion of theincoming light and the polarization-rotated second portion of theincoming light at said any given one of the plurality of ring resonatorphotodetectors 161-1 to 161-6. After the timing-skew management system165 operates to electronically compensate for the temporal difference inphotocurrent generation by each of the plurality of ring resonatorphotodetectors 161-1 to 161-6, the photocurrents generated by each ofthe plurality of ring resonator photodetectors 161-1 to 161-6 aretransmitted to photocurrent processing circuitry 167 to decode thephotocurrents into digital data patterns as conveyed by the incomingoptical signal. In some embodiments where light from differentpolarizations is made to have a same polarization and is combined into asingle waveguide, or where light from different polarizations is made tohave a same polarization and is combined into a single photodetector orset of photodetectors, there may be a time delay difference(timing-skew) between the optical signals from each polarization state.After converting the combined optical signal into an electronic signalwith a photodetector or set of photodetectors, the timing-skew manifestsitself as a notch filter around an electrical radiofrequency componentof the baseband signal. The center frequency of the notch filter isdependent on the magnitude of the timing-skew, and the depth of thenotch filter is determined by the relative split in optical powerbetween the two polarization states. For a digital communicationsapplication, this notch filter results in increased inter-symbolinterference (ISI). The timing-skew management system 165 is configuredto detect the presence of the timing-skew, determine the magnitude ofthe timing-skew, and compensate for the timing-skew in thephotocurrent-based signals that are transmitted to the photocurrentprocessing circuitry 167.

In some embodiments, the temporal difference (timing-skew) inphotocurrent generation by a given one of the plurality of ringresonator photodetectors 161-1 to 161-6 caused by a difference inarrival time of the first portion of the incoming light and thepolarization-rotated second portion of the incoming light at the givenone of the plurality of ring resonator photodetectors 161-1 to 161-6 isreduced by reducing a difference in optical travel distance to the givenone of the plurality of ring resonator photodetectors 161-1 to 161-6that is traveled by the first portion of the incoming light and thepolarization-rotated second portion of the incoming light. FIGS. 2A, 2B,and 2C illustrate examples of how to reduce the difference in opticaltravel distance through a same optical waveguide to a given one of aplurality of photodetectors that is traveled by the first portion of theincoming light and the polarization-rotated second portion of theincoming light, where the first portion of the incoming light and thepolarization-rotated second portion of the incoming light travel inopposite directions through the same optical waveguide, such as in theelectro-optic receiver 150 of FIG. 1B.

In various embodiments, the PSR 156, and any of the PSR's referred toherein, is configured in a manner that provides for: 1) reception of twopolarizations of incoming light (TE and TM), 2) rotation of one of thepolarizations of the incoming light to the other polarization (eitherrotation of TE to TM, or rotation of TM to TE), 3) outputting of theportion of the incoming light having the polarization that was notrotated to a first output optical waveguide, and 4) outputting of theportion of the incoming light having the polarization that was rotatedto a second output optical waveguide. In it should be understood that invarious embodiments, the polarization rotation andoriginal-polarization-based splitting of the incoming light performed bythe PSR 156, and any of the PSR's referred to herein, can be donesimultaneously or sequentially within the PSR (e.g., with thepolarization rotation happening either before, after, or in conjunctionwith the original-polarization-based splitting of the incoming light).

FIG. 1C shows an example configuration of a PSR 171, in accordance withsome embodiments. It should be understood that the example PSR 171 canbe used for the PSR 156 and/or any of the PSR's referred to herein.Also, it should be understood that the PSR 171 is provided by way ofexample and in no way limits how the PSR 156 and/or any of PSR'sreferred to herein can be configured in various embodiments. The PSR 171includes a first optical waveguide 172 and a second optical waveguide173. FIG. 1D shows a vertical cross-section view through the example PSR171, referenced as View A-A in FIG. 1C, in accordance with someembodiments. In some embodiments, the first optical waveguide 172 is asilicon nitride optical waveguide, and the second optical waveguide 173is a silicon optical waveguide. In some embodiments, the PSR 171 isformed on a buried oxide (BOX) layer 175 that is disposed over asubstrate 174. In some embodiments, the first optical waveguide 173 andthe second optical waveguide 172 are formed within an optical cladding176. In some embodiments, the optical cladding 176 is silicon dioxide.The first optical waveguide 172 is vertical separated from the secondoptical waveguide 173 by a layer of the optical cladding, as indicatedby arrow 177.

The first optical waveguide 172 includes an input section 172A connectedto receive incoming light that includes both the TE and TMpolarizations. In some embodiments, the input section 172A is configuredas an inverse taper to convert a spot size of the incoming light to theoptical mode of the first optical waveguide 172. After the input section172A (with respect to the light propagation direction) the first opticalwaveguide 172 includes a rotation/splitting section 172B. In someembodiments, the rotation/splitting section 172B has a substantiallylinear shape. After the rotation/splitting section 172B, the firstoptical waveguide 172 includes an output section 172C that is opticallyconnected to a first optical output 172D of the PSR 171. The firstoptical waveguide 172 is configured such that a portion of the incominglight having a first polarization (TE or TM) travels through the firstoptical waveguide 172 to the first optical output 172D of the PSR 171 ina substantially unchanged manner. The example PSR 171 shows the TEpolarization of the incoming light traveling through the first opticalwaveguide 172 to the first optical output 172D of the PSR 171 in asubstantially unchanged manner.

The second optical waveguide 173 includes a rotation/splitting section173A that is configured to evanescently in-couple the TM polarization ofthe incoming light from the rotation/splitting section 172B of the firstoptical waveguide 172 and simultaneously rotate the in-coupled TMpolarization to the TE polarization. To accomplish this, therotation/splitting section 173A of the second optical waveguide 173 hasan inverse taper configuration that positioned off-center (having alateral offset in a direction perpendicular to the light propagationdirection) with respect to the rotation/splitting section 172B of thefirst optical waveguide 172. The lateral offset of therotation/splitting section 173A of the second optical waveguide 173 withrespect to the rotation/splitting section 172B of the first opticalwaveguide 172 serves to break horizontal and vertical symmetry so as torotate the TM0 mode in the rotation/splitting section 172B of the firstoptical waveguide 172 to a rotated TE0 mode and couple this rotated TE0mode into the rotation/splitting section 173A of the second opticalwaveguide 173. The rotated TE0 mode is conveyed through an outputsection 173B of the second optical waveguide 173 to a second opticaloutput 173C of the PSR 171. While the example PSR 171 shows the TEpolarization of the incoming light traveling through the first opticalwaveguide 172 to the first optical output 172D of the PSR 171 in asubstantially unchanged manner, and shows the TM polarization of theincoming light being rotated to the TE polarization in route to thesecond optical output 173C of the PSR 171, other embodiments of the PSR171 are configured to have the TM polarization of the incoming lighttravel through the first optical waveguide 172 to the first opticaloutput 172D of the PSR 171 in a substantially unchanged manner, with theTE polarization of the incoming light rotated to the TM polarization inroute to the second optical output 173C of the PSR 171.

FIG. 1E shows an example configuration of a PSR 181, in accordance withsome embodiments. It should be understood that the example PSR 181 canbe used for the PSR 156 and/or any of the PSR's referred to herein.Also, it should be understood that the PSR 181 is provided by way ofexample and in no way limits how the PSR 156 and/or any of PSR'sreferred to herein can be configured in various embodiments. The PSR 181is a broadband PSR that implements rib-type optical waveguides. The PSR181 includes a first branch 183 and a second branch 185. The PSR 181 isconfigured as an optical waveguide system that includes a first branchslab waveguide 187, a first branch rib waveguide 189, a second branchslab waveguide 188, and a second branch rib waveguide 190. FIG. 1F showsa vertical cross-section view through the example PSR 181, referenced asView A-A in FIG. 1E, in accordance with some embodiments. FIG. 1G showsa vertical cross-section view through the example PSR 181, referenced asView B-B in FIG. 1E, in accordance with some embodiments. In someembodiments, the first branch slab waveguide 187, the second branch slabwaveguide 188, the first branch rib waveguide 189, and the second branchrib waveguide 190 are integrally formed as a monolithic opticalwaveguide structure, in which the first branch slab waveguide 187, thesecond branch slab waveguide 188, the first branch rib waveguide 189,and the second branch rib waveguide 190 form different parts of themonolithic optical waveguide structure. In some embodiments, themonolithic optical waveguide structure is formed as a silicon opticalwaveguide. In some embodiments, the monolithic optical waveguidestructure is formed as a silicon nitride optical waveguide. In someembodiments, the PSR 181 is formed on a BOX layer 191 that is disposedover a substrate 192. In some embodiments, the monolithic opticalwaveguide structure that includes the first slab waveguide 187, thesecond branch slab waveguide 188, the first branch rib waveguide 189,and the second branch rib waveguide 190 is formed within an opticalcladding 193. In some embodiments, the optical cladding 193 is silicondioxide.

The first branch rib waveguide 189 includes an input section 189A thathas a substantially linear shape, followed by a tapered section 189B,followed by an output section 189C (with respect to a light propagationdirection through the PSR 181). The first branch slab waveguide 187includes tapered input section 187A, followed by a tapered section 187B(corresponding to the rib tapered section 189B), followed by an outputsection 187C (corresponding to the rib output section 189C). The secondbranch rib waveguide 190 includes a tapered section 190A, followed by anoutput section 190B (with respect to a light propagation directionthrough the PSR 181). The second branch slab waveguide 188 includestapered section 188A (corresponding to the rib tapered section 190A,followed by an output section 188B (corresponding to the rib outputsection 190B). In some embodiments, a portion of the output section 188Bof the second branch slab waveguide 188 located between the first branchrib waveguide 189 and the second branch rib waveguide 190 has anincreasing width along the light propagation direction to provide foreasier optical routing of the outputs of the first branch 183 and thesecond branch 185 to separate output optical waveguides.

The input section 189A and the tapered section 189B of the first branchrib waveguide 189, and the tapered input section 187A and the taperedsection 187B of the first branch slab waveguide 187, collectivelyfunction as a polarization rotator. The output section 189C of the firstbranch rib waveguide 189 and the output section 187C of the first branchslab waveguide 187, and the tapered section 190A of the second branchrib waveguide 190 and the tapered section 188A of the second branch slabwaveguide 188, collectively function as a polarization splitter. In thismanner, the TE0 polarization of the incoming light is transmitted in asubstantially unchanged manner through the first branch 183 to a firstoptical output 195 of the PSR 181. The TM0 polarization of the incominglight is rotated to a TE1 polarization and then to a TE0 polarization asthis portion of the incoming light is transmitted through the firstbranch 183 and is optically coupled into the second branch 185 in routeto a second optical output 197 of the PSR 181. Alternatively, in someother embodiments, the PSR 181 is configured to pass through the TMpolarization of the incoming light in a substantially unchanged mannerand rotate/split the TE polarization of the incoming light to outgoingTM polarized light.

It should be understood that the PSR 171 and the PSR 181 are provided asexamples of how the PSR's described herein may be implemented in someexample embodiments. It should also be understood that in someembodiments, any of the PSR's described herein can be implemented as adual-polarization grating coupler, such as described with regard to FIG.1A. It should be understood that the example PSR 171 and the example PSR181 do not limit in any way how the various PSR's described herein canbe implemented in various embodiments. Any of the PSR's described hereincan be implemented in different ways so long as one of the two inputpolarizations (TE or TM) of incoming light received by the PSR isrotated to the other polarization and is directed to one of two outputsof the PSR, with the non-rotated polarization of the incoming lightbeing directed to another of the two outputs of the PSR.

FIG. 2A shows an example of a WDM optical receiver 201 that includesmultiple ring resonator photodetectors 203-1 to 203-6 (one for eachwavelength channel) positioned along an optical waveguide 205 that isconfigured to extend in a continuous, loop-like configuration from afirst end 205A to a second end 205B, in accordance with someembodiments. Each of the multiple ring resonator photodetectors 203-1 to203-6 is electrically connected to transmit detected photocurrent to acorresponding one of multiple receiver circuits 207-1 to 207-6. Each ofthe multiple ring resonator photodetectors 203-1 to 203-6 is positionedto evanescently in-couple light from the optical waveguide 205 that hasa substantially same wavelength as the operating resonant wavelength ofthe particular ring resonator photodetector. In some embodiments, eachof the ring resonator photodetectors 203-1 to 203-6 is configured tohave an annular-shape or disc-shape with an outer diameter of less thanabout 50 micrometers. In some embodiments, each of the ring resonatorphotodetectors 203-1 to 203-6 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about micrometers. Theoptical waveguide 205 has a center location 205C that is about halfwaybetween the first end 205A and the second end 205B of the opticalwaveguide 205. In the example of FIG. 2A, all of the multiple ringresonator photodetectors 203-1 to 203-6 are positioned along a same halfof the optical waveguide 205, which causes an increased optical pathlength mismatch between the different ring resonator photodetectors203-1 to 203-6. Therefore, in the example of FIG. 2A, there is a largetemporal difference (timing-skew) in photocurrent generation by a givenone of the multiple ring resonator photodetectors 203-1 to 203-6 that iscaused by a difference in arrival time of the first portion of theincoming light and the polarization-rotated second portion of theincoming light at the given one of the multiple ring resonatorphotodetectors 203-1 to 203-6.

FIG. 2B shows a WDM optical receiver 201A that is modified version ofthe WDM optical receiver 201 of FIG. 2A, in accordance with someembodiments. In the WDM optical receiver 201A, a first half of themultiple ring resonator photodetectors 203-1 to 203-3 are positionedalong a first half of the optical waveguide 205 extending from the firstend 205A to the center location 205C of the optical waveguide. The firsthalf of the ring resonator photodetectors 203-1 to 203-3 are alsopositioned close to the center location 205C of the optical waveguide205. Similarly, in the WDM optical receiver 201A, a second half of themultiple ring resonator photodetectors 203-4 to 203-6 are positionedalong a second half of the optical waveguide 205 extending from thesecond end 205B to the center location 205C of the optical waveguide205. The second half of the ring resonator photodetectors 203-1 to 203-3are also positioned close to the center location 205C of the opticalwaveguide 205. In the optical receiver 201A, the ring resonatorphotodetectors 203-1 to 203-6 are distributed evenly about the centerlocation 205C of the optical waveguide 205 and as close as possible tothe center location 205C of the optical waveguide 205. It should beunderstood that centering the multiple ring resonator photodetectors203-1 to 203-6 on the center location 205C of the optical waveguide 205minimizes a difference in optical travel distance to a given one of themultiple ring resonator photodetectors 203-1 to 203-6 that is traveledby the first portion of the incoming light and the polarization-rotatedsecond portion of the incoming light, which in turn minimizes thetiming-skew between the first portion of the incoming light and thepolarization-rotated second portion of the incoming light at the givenone of the multiple ring resonator photodetectors 203-1 to 203-6. Theoptical receiver 201A minimizes the residual path length differencebetween each polarization of each of the wavelength channel signals inthe incoming optical signal.

FIG. 2C shows a WDM optical receiver 201B that is modified version ofthe WDM optical receiver 201A of FIG. 2B, in accordance with someembodiments. In the WDM optical receiver 201B, the optical waveguide 205is replaced by an optical waveguide 209 that has a first end 209A, asecond end 209B, and a center location 209C. The optical waveguide 209also includes an extra waveguide section 209D that is configured so thatthe center location 209C of the optical waveguide 209 is positionedalong a linear stretch of the optical waveguide 209 that is long enoughto accommodate a linear positioning of the multiple ring resonatorphotodetectors 203-1 to 203-6 along the linear stretch of the opticalwaveguide 209, with the multiple ring resonator photodetectors 203-1 to203-6 centered on the center location 205C of the optical waveguide 205.Having the multiple ring resonator photodetectors 203-1 to 203-6centered on the center location 209C of the optical waveguide 205provides for minimization of the difference in optical travel distanceto a given one of the multiple ring resonator photodetectors 203-1 to203-6 that is traveled by the first portion of the incoming light andthe polarization-rotated second portion of the incoming light, which inturn minimizes the timing-skew between the first portion of the incominglight and the polarization-rotated second portion of the incoming lightat the given one of the multiple ring resonator photodetectors 203-1 to203-6. It should be appreciated that having the multiple ring resonatorphotodetectors 203-1 to 203-6 positioned in the linear arrangementwithin the WDM optical receiver 201B provides for placement of thereceiver circuits 207-1 to 207-6 next to each other on the chip, whichopens up other areas of the chip for implementation of other photonicand/or electronic circuitry. The optical receiver 201B also minimizesthe residual path length difference between each of the signals of eachpolarization.

FIG. 3 shows an example configuration of an electro-optic receiver 300implemented within a PIC 302, in accordance with some embodiments Likethe electro-optic receiver 150 of FIG. 1B, the electro-optic receiver300 receives the incoming optical signal from the opticalfiber/waveguide 152 through the optical coupler 153, as indicated byarrow 154. The incoming optical signal is conveyed from the opticalcoupler 153 through the optical waveguide 155 to the optical input ofthe PSR 156. As with the electro-optic receiver 150 of FIG. 1B, the PSR156 is configured to direct a first portion of the incoming light havinga first polarization through the first optical output 156B of the PSR156. The PSR 156 is also configured to rotate a polarization of a secondportion of the incoming light from a second polarization to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The PSR 156is configured to direct the polarization-rotated second portion of theincoming light through the second optical output 156C of the PSR 156.The first end 157A of the optical waveguide 157 is optically connectedto the first optical output 156B of the PSR 156. The second end 157B ofthe optical waveguide 157 is optically connected to the second opticaloutput 156C of the PSR 156. The optical waveguide 157 of theelectro-optic receiver 300 is like that of the electro-optic receiver150 of FIG. 1B. The optical waveguide 157 has the continuous, loop-likestructure. In this manner, the first portion of the incoming lighttravels from the first optical output 156A of the PSR 156 through theoptical waveguide 157 in a first direction, as indicated by arrows 158.Also, the polarization-rotated second portion of the incoming lighttravels from the second optical output 156C of the PSR 156 through theoptical waveguide 157 in a second direction, as indicated by arrows 159,that is opposite the first direction.

The electro-optic receiver 300 includes a plurality of ring resonators301-1 to 301-3 positioned alongside the optical waveguide 157 and withinan evanescent optical coupling distance of the optical waveguide 157.The example configuration of the electro-optic receiver 300 includesthree ring resonators 301-1 to 301-3 for description purposes. It shouldbe understood that in various embodiments, the electro-optic receiver300 includes either less than three or more than three ring resonatorspositioned alongside the optical waveguide 157 and within an evanescentoptical coupling distance of the optical waveguide 157. There is nolimit on the number of ring resonators that (e.g., 301-1 to 301-3) thatcan be positioned along the optical waveguide 157, so long as the ringresonators and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. In some embodiments, the ringresonators 301-1 to 301-3 are implemented as annular-shaped waveguideshaving circuitous configuration, e.g., circular, oval, race-track, oranother arbitrary circuitous shape. In some embodiments, the ringresonators 301-1 to 301-3 are implemented as circular discs. The ringresonators 301-1 to 301-3 are formed of a material through which lightcan be in-coupled, out-coupled, and guided. Each of the ring resonators301-1 to 301-3 is formed within a surrounding material that has anoptical index of refraction sufficiently different from that of the ringresonators 301-1 to 301-3 to enable guiding of light within the ringresonators 301-1 to 301-3 and around the circuitous path defined by eachof the ring resonators 301-1 to 301-3. In some embodiments, each of thering resonators 301-1 to 301-3 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 50 micrometers. Insome embodiments, each of the ring resonators 301-1 to 301-3 isconfigured to have an annular-shape or disc-shape with an outer diameterof less than about 10 micrometers.

Each of the plurality of ring resonators 301-1 to 301-3 is configured tooperate at a respective resonant wavelength λ₁ to λ₃. In this manner,the first portion of the incoming light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the plurality of ring resonators 301-1 to 301-3 optically couplesinto the given one of the plurality of ring resonators 301-1 to 301-3 ina first propagation direction, and the polarization-rotated secondportion of the incoming light having a wavelength substantially equal tothe respective resonant wavelength of the given one of the plurality ofring resonators 301-1 to 301-3 optically couples into the given one ofthe plurality of ring resonators 301-1 to 301-3 in a second propagationdirection opposite the first propagation direction. For example, thefirst portion of the incoming light having a particular wavelengthoptically couples into the ring resonator 301-x operating at theparticular wavelength and propagates in a counter-clockwise directionwithin the ring resonator 301-x. The polarization-rotated second portionof the incoming light having a particular wavelength optically couplesinto the ring resonator 301-x operating at the particular wavelength andpropagates in a clockwise direction within the ring resonator 301-x.

The electro-optic receiver 300 includes a plurality of output opticalwaveguides 303-1 to 303-3 positioned within an evanescent opticalcoupling distance of the plurality of ring resonators 301-1 to 301-3,respectively. Each of the plurality of output optical waveguides 303-1to 303-3 includes a coupling section 303A-1 to 303A-3, respectively.Each of the plurality of output optical waveguides 303-1 to 303-3includes a short section 303B-1 to 303B-3, respectively. Each of theplurality of output optical waveguides 303-1 to 303-3 includes a longsection 303C-1 to 303C-3, respectively. The number of output opticalwaveguides 303-1 to 303-3 is equal to the number of ring resonators301-1 to 301-3. Therefore, as the number of ring resonators changes invarious embodiments, so does the number of output optical waveguides.The coupling section 303A-1 to 303A-3 is positioned to evanescentlyin-couple light from a corresponding one of the plurality of ringresonators 301-1 to 301-3. In this manner, each of the ring resonators301-1 to 301-3 operates to transfer a particular wavelength of the firstportion of the incoming light and the polarization-rotated secondportion of the incoming light from the optical waveguide 157 to thecorresponding one of the output optical waveguides 303-1 to 303-3. Thefirst portion of the incoming light that propagates in thecounter-clockwise direction within the ring resonators 301-1 to 301-3 isoptically coupled through the coupling section 303A-1 to 303A-3,respectively, and into the long section 303C-1 to 303C-3, respectively,as indicated by arrows 307-1 to 307-3, respectively. Thepolarization-rotated second portion of the incoming light thatpropagates in the clockwise direction within the ring resonators 301-1to 301-3 is optically coupled through the coupling section 303A-1 to303A-3, respectively, and into the short section 303B-1 to 303B-3,respectively, as indicated by arrows 309-1 to 309-3, respectively.

The output optical waveguides 303-1 to 303-3 are formed of a materialthrough which light can be in-coupled, out-coupled, and guided. Each ofthe output optical waveguides 303-1 to 303-3 is formed within asurrounding material that has an optical index of refractionsufficiently different from that of the output optical waveguides 303-1to 303-3, respectively, to enable guiding of light within the outputoptical waveguides 303-1 to 303-3. In some embodiments, the outputoptical waveguides 303-1 to 303-3 are implemented to have a rack-tracktype shape. However, it should be understood that in other embodiments,the output optical waveguides 303-1 to 303-3 can be implemented to havean arbitrary shape, so long as they include the coupling section 303A-1to 303A-3, respectively, and the short section 303B-1 to 303B-3,respectively, and the long section 303C-1 to 303C-3, respectively.

The electro-optic receiver 300 includes a plurality of photodetectors305-1 to 305-3 respectively associated with the plurality of ringresonators 301-1 to 301-3. Therefore, as the number of ring resonatorschanges in various embodiments, so does the number of photodetectors.The short section 303B-1 to 303B-3 of the output optical waveguides303-1 to 303-3 extends from a first end of the corresponding couplingsection 303A-1 to 303A-3 to the corresponding one of the plurality ofphotodetectors 305-1 to 305-3. The long section 303C-1 to 303C-3 of theoutput optical waveguides 303-1 to 303-3 extends from a second end ofthe corresponding coupling section 303A-1 to 303A-3 to the correspondingone of the plurality of photodetectors 305-1 to 305-3. A length of thelong section 303C-1 to 303C-3 and a length of the short section 303B-1to 303B-3 within a given one of the output optical waveguides 303-1 to303-3 are defined to reduce a difference in arrival time of the firstportion of the incoming light and the polarization-rotated secondportion of the incoming light at the corresponding one of thephotodetectors 305-1 to 305-3 to which the long section 303C-1 to 303C-3and the short section 303B-1 to 303B-3 are optically connected. Becausethe distance along the optical waveguide 157 from the second end 157B ofthe optical waveguide 157 to each of the ring resonators 301-1 to 301-3is different, the length of the long section 303C-1 to 303C-3 isdifferent for each of the output optical waveguides 303-1 to 303-3. Insome embodiments, the length of the long section 303C-1 to 303C-3decreases as a distance between the corresponding one of the pluralityof ring resonators 301-1 to 301-3 and the midpoint of the opticalwaveguide 157 decreases, where the midpoint of the optical waveguide 157is about halfway between the first end 157A and the second end 157B ofthe optical waveguide 157.

The electro-optic receiver 300 also includes the timing-skew managementsystem 165 configured to electronically compensate for a temporaldifference in photocurrent generation by a given one of the plurality ofphotodetectors 305-1 to 305-3 caused by the difference in arrival timeof the first portion of the incoming light and the polarization-rotatedsecond portion of the incoming light at the given one of the pluralityof photodetectors 305-1 to 305-3. In some embodiments, each of thephotodetectors 305-1 to 305-3 is a linear photodetector, with the shortsection 303B-1 to 303B-3 of the corresponding one of the output opticalwaveguides 303-1 to 303-3 optically connected to a first end of thelinear photodetector, and with the long section 303C-1 to 303C-3 of thecorresponding one of the output optical waveguides 303-1 to 303-3optically connected to a second end of the linear photodetector.

FIG. 4 shows a diagram of an example linear photodetector 400, inaccordance with some embodiments. In some embodiments, a separateinstance of the linear photodetector 400 is used for each of thephotodetectors 305-1 to 305-3 in the electro-optic receiver 300. Thelinear photodetector 400 is shown as a PIN type of photodetector thatincludes a P doped region 401, an intrinsic region 403, and an N dopedregion 405. The intrinsic region 403 is positioned between the P dopedregion 401 and the N doped region 405. In some embodiments, the P dopedregion 401 and the N doped region 405 are switched. In some embodiments,the intrinsic region 403 is an optical waveguide through which lightthat is to be detected is directed. In some embodiments, at least aportion of the P doped region 401 and at least a portion of the N dopedregion 405 is formed within the optical waveguide. During operation thephotodetector 400 is reverse-biased so that charge carriers generated byphoto-absorption within the intrinsic region 403 are swept intoelectrical contacts 407 connected along the length of the photodetector400.

The linear photodetector 400 enables the incoming light from eachpolarization to be detected independently. More specifically, the firstportion of the incoming light having the first polarization is inputthrough a first end of the photodetector 400, and thepolarization-rotated second portion of the incoming light having thefirst polarization (but corresponding to the incoming light that had thesecond polarization) is input through a second end of the photodetector400. Due to photo-absorption along the length of the linearphotodetector 400, the intensity of the first portion of the incominglight having the first polarization as input through the first end ofthe photodetector 400 decays exponentially in accordance with thephoto-absorption coefficient as the light travels along the length ofthe photodetector 400 in the first direction, as indicated by arrow 409.Similarly, the intensity of the polarization-rotated second portion ofthe incoming light having the first polarization as input through thesecond end of the photodetector 400 decays exponentially in accordancewith the photo-absorption coefficient as the light travels along thelength of the photodetector 400 in the second direction, as indicated byarrow 411. Therefore, a majority of the first portion of the incominglight having the first polarization is absorbed within a first half 413of the linear photodetector 400, and a majority of polarization-rotatedsecond portion of the incoming light having the first polarization isabsorbed within a second half 415 of the linear photodetector 400. Insome embodiments, the electrical contacts 407 along the first half 413of the photodetector 400 are segmented and connected to a firstreverse-biasing circuit and a first receive circuit, and the electricalcontacts 407 along the second half 415 of the photodetector 400 aresegmented and connected to a second reverse-biasing circuit and a secondreceive circuit. In these embodiments, comparison of the photocurrentmeasured by the first receive circuit to the photocurrent measured bythe second receive circuit provides for determination of the relativeoptical power split between different polarizations within the incomingoptical signal, given that the polarization-rotated second portion ofthe incoming signal actually corresponds to the second polarizationwithin the incoming optical signal.

The electro-optic receiver 300 of FIG. 3 represents an embodiment inwhich the input optical fiber/waveguide 152 transmits an input opticalsignal with no polarization control into the PIC 302. The opticalcoupler 153 couples the incoming light of the input optical signal ontothe waveguide 155 on the chip. In some embodiments, the PSR 156 splitsthe input optical signal polarizations and transmits the twopolarizations into two separate waveguides within the PSR 156. One ofthese two waveguides within the PSR 156 passes through a polarizationrotator, so that its polarization is rotated into the same state as theother waveguide within the PSR 156. The two waveguides within the PSR156 are optically connected to the first end 157A and the second end157B, respectively, of the optical waveguide 157 (loop) which isevanescently coupled to the ring resonators 301-1 to 301-3. In someembodiments, the ring resonators 301-1 to 301-3 are respectivelyreplaced by passive ring filters. Each of the ring resonators 301-1 to301-3 (or passive ring filters) is designed to route the light from asingle wavelength channel, within a narrow wavelength range, to thecorresponding output optical waveguide 303-1 to 303-3 that is opticallyconnected to the corresponding photodetector 305-1 to 305-3 (e.g.,linear photodetector 400).

In the electro-optic receiver 300 of FIG. 3 , light that is coupled fromthe optical waveguide 157 into the ring resonator 301-1 to 301-3 (orpassive ring filter) from the right (light traveling from the second end157B toward the first end 157A of the optical waveguide 157) is routedclockwise through the ring resonator 301-1 to 301-3 (or passive ringfilter) and is coupled into the coupling section 303A-1 to 303A-3 of thecorresponding output optical waveguide 303-1 to 303-3, where it passesthrough the short section 303B-1 to 303B-3 of the corresponding outputoptical waveguide 303-1 to 303-3 and into the second end of thephotodetector 305-1 to 305-3. Conversely, light that is coupled from theoptical waveguide 157 into the ring resonator 301-1 to 301-3 (or passivering filter) from the left (light traveling from the first end 157Atoward the second end 157B of the optical waveguide 157) is routedcounter-clockwise through the ring resonator 301-1 to 301-3 (or passivering filter) and is coupled into the coupling section 303A-1 to 303A-3of the corresponding output optical waveguide 303-1 to 303-3, where itpasses through the long section 303C-1 to 303C-3 of the correspondingoutput optical waveguide 303-1 to 303-3 and into the first end of thephotodetector 305-1 to 305-3. The long section 303C-1 to 303C-3 of theoutput optical waveguide 303-1 to 303-3 functions as an optical delayline. The long section 303C-1 to 303C-3 of the output optical waveguide303-1 to 303-3 is defined so that the two polarizations of the incomingoptical signal from a given wavelength channel in the opticalfiber/waveguide 152 (after splitting, polarization rotation, andwaveguide routing), enter a given photodetector 305-1 to 305-3 (definedas the linear photodetector 400) from opposite directions at about thesame time, which reduces timing-skew between the two polarizations ofthe incoming optical signal at the given photodetector 305-1 to 305-3and correspondingly makes electronic timing-skew management possible (oreasier) to implement, and may eliminate the need for timing-skewmanagement altogether.

Additionally, in some embodiments of the electro-optic receiver 300 ofFIG. 3 , the photocurrent from a given photodetector 305-1 to 305-3 isused as a feedback signal to control alignment between the resonantwavelength of the corresponding ring resonator 301-1 to 301-3 and thewavelength of a given data channel in the incoming optical signal. Forexample, in some embodiments, the electro-optic receiver 300 includes acontrol circuit that tunes the ring resonance wavelength of the ringresonator 301-1 to 301-3 using a thermal tuner and/or a diode built intothe ring resonator 301-1 to 301-3, in order to optimize the opticalpower reaching the photodetector 305-1 to 305-3. Also, in someembodiments, the electro-optic receiver 300 includes the VOA 163, asdescribed with regard to the electro-optic receiver 150 of FIG. 1B, foruse at startup of the electro-optic receiver 300.

FIG. 5 shows a flowchart of a method for operating a photonic circuit,in accordance with some embodiments. In some embodiments, the method ofFIG. 5 is practiced using the electro-optic receivers 150 and/or 300.The method includes an operation 501 for receiving incoming lightthrough an optical input port of the photonic circuit. A first portionof the incoming light has a first polarization and a second portion ofthe incoming light has a second polarization. The method also includesan operation 503 for splitting the first portion of the incoming lightfrom the second portion of the incoming light, such as by using the PSR156. The method also includes an operation 505 for directing the firstportion of the incoming light through a first end of an opticalwaveguide, such as the through the first end 157A of the opticalwaveguide 157. The method also includes an operation 507 for rotatingthe second polarization of the second portion of the incoming light tothe first polarization so that the second portion of the incoming lightis a polarization-rotated second portion of the incoming light, such asdone by the PSR 156. The method also includes an operation 509 fordirecting the polarization-rotated second portion of the incoming lightthrough a second end of the optical waveguide, such as the through thesecond end 157B of the optical waveguide 157, where the opticalwaveguide 157 extends in a continuous manner from the first end 157A tothe second end 157B. In some embodiments, the operations 503, 505, 507,and 509 are done is an essentially simultaneous manner. The method alsoincludes an operation 511 for operating a plurality of ring resonators,e.g., 161-1 to 161-6 and 301-1 to 301-3, to evanescently in-couple lightfrom the optical waveguide 157, wherein each of the plurality of ringresonators is operated at a respective resonant wavelength to in-coupleboth the first portion of the incoming light having the respectiveresonant wavelength and the polarization-rotated second portion of theincoming light having the respective resonant wavelength.

In some embodiments, the method also includes transmitting the firstportion of the incoming light and the second portion of the incominglight through a variable optical attenuator, such as the VOA 163, andcontrolling the variable optical attenuator to attenuate an opticalpower of first portion of the incoming light and the second portion ofthe incoming light during a time when the plurality of ring resonators,e.g., 161-1 to 161-6 and 301-1 to 301-3, are being tuned to theirrespective resonant wavelengths. In these embodiments, the method alsoincludes controlling the variable optical attenuator to not attenuatethe optical power of first portion of the incoming light and the secondportion of the incoming light during a time when the plurality of ringresonators are operating at their respective resonant wavelengths, e.g.,during normal operation.

In some embodiments, each of the plurality of ring resonators, e.g.,161-1 to 161-6, includes a respective photodetector. In someembodiments, the method includes optically coupling light from each ofthe plurality of ring resonators, e.g., 301-1 to 301-3, into acorresponding one of a plurality of output optical waveguides, e.g.,303-1 to 303-3, such that the first portion of the incoming light istransmitted through a long section, e.g., 303C-1 to 3030C-3, of thecorresponding one of the plurality of output optical waveguides, andsuch that the polarization-rotated second portion of the incoming lightis transmitted through a short section, e.g., 303B-1 to 303B-3, of thecorresponding one of the plurality of output optical waveguides. Inthese embodiments, the method also includes operating a photodetector,e.g., 305-1 to 305-3, to detect the first portion of the incoming lightat an output end of the long section of the corresponding one of theplurality of output optical waveguides, and to detect thepolarization-rotated second portion of the incoming light at an outputend of the short section of the corresponding one of the plurality ofoutput optical waveguides.

FIG. 6 shows an example configuration of an electro-optic receiver 600implemented within a PIC 601, in accordance with some embodiments. Theelectro-optic receiver 600 includes a PSR 613 that has an optical input613A optically connected to receive incoming light from an opticalcoupler 615, by way of an optical waveguide 614. In some embodiments,the optical input 613A of the PSR 156 is directly optically coupled tothe optical coupler 615, such that the optical waveguide 155 is notrequired. In some embodiments, the optical coupler 615 is implemented asan edge coupler. However, in other embodiments, the optical coupler 615is implemented as a vertical grating coupler, or as another type ofoptical coupling device that provides for optical coupling of the PIC601 to an optical fiber/waveguide 617. Incoming light is transmittedfrom the optical fiber/waveguide 617 into the optical coupler 615, asindicated by arrow 616. The PSR 613 has a first optical output 613B anda second optical output 613C. The PSR 613 is configured to direct afirst portion of the incoming light having a first polarization (TE orTM) through the first optical output 613B. The PSR 613 is alsoconfigured to rotate a polarization of a second portion of the incominglight from a second polarization (opposite of the first polarization) tothe first polarization. In this manner, the PSR 613 turns the secondportion of the incoming light into a polarization-rotated second portionof the incoming light. The PSR 613 is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output 613C.

The electro-optic receiver 600 includes a first optical waveguide 603optically connected to the first optical output 613B of the PSR 613. Theelectro-optic receiver 600 also includes a second optical waveguide 605optically connected to the second optical output 613C of the PSR 613. Inthe electro-optic receiver 600, the first optical waveguide 603 and thesecond optical waveguide 605 are not optically connected/coupled to eachother. The first optical waveguide 603 and the second optical waveguide605 are formed of a material through which light can be in-coupled,out-coupled, and guided. Each of the first optical waveguide 603 and thesecond optical waveguide 605 is formed within a surrounding materialthat has an optical index of refraction sufficiently different from thatof the first optical waveguide 603 and the second optical waveguide 605,respectively, to enable guiding of light within the first opticalwaveguide 603 and the second optical waveguide 605. In some embodiments,first optical waveguide 603 and the second optical waveguide 605 areformed of a same material. The first portion of the incoming light istransmitted through the first optical output 613B of the PSR 613 andinto the first optical waveguide 603, and travels along the firstoptical waveguide 603, as indicated by arrows 604. Thepolarization-rotated second portion of the incoming light is transmittedthrough the second optical output 613C of the PSR 613 and into thesecond optical waveguide 605, and travels along the second opticalwaveguide 605, as indicated by arrows 606.

The electro-optic receiver 600 includes a first plurality of ringresonators 607-1 to 607-3 positioned along the first optical waveguide603 and within an evanescent optical coupling distance of the firstoptical waveguide 603. While the example electro-optic receiver 600shows three ring resonators 607-1 to 607-3 for purposes of description,it should be understood that there is no limit on the number of thefirst plurality of ring resonators that can be positioned along thefirst optical waveguide 603, so long as the first plurality of ringresonators and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. Each of the ring resonators607-1 to 607-3 is configured to operate at a respective resonantwavelength λ₁ to λ₃, such that the first portion of the incoming lighthaving a wavelength (λ₁, λ₂, or λ₃) substantially equal to therespective resonant wavelength (λ₁, λ₂, or λ₃) of a given one of thering resonators 607-1 to 607-3 optically couples into the given one ofthe ring resonators 607-1 to 607-3. In some embodiments, the ringresonators 607-1 to 607-3 are implemented as annular-shaped waveguideshaving circuitous configuration, e.g., circular, oval, race-track, oranother arbitrary circuitous shape. In some embodiments, the ringresonators 607-1 to 607-3 are implemented as circular discs. The ringresonators 607-1 to 607-3 are formed of a material through which lightcan be in-coupled, out-coupled, and guided. Each of the ring resonators607-1 to 607-3 is formed within a surrounding material that has anoptical index of refraction sufficiently different from that of the ringresonators 607-1 to 607-3 to enable guiding of light within the ringresonators 607-1 to 607-3 and around the circuitous path defined by eachof the ring resonators 607-1 to 607-3. In some embodiments, each of thering resonators 607-1 to 607-3 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 50 micrometers. Insome embodiments, each of the ring resonators 607-1 to 607-3 isconfigured to have an annular-shape or disc-shape with an outer diameterof less than about 10 micrometers.

The electro-optic receiver 600 includes a second plurality of ringresonators 609-1 to 609-3 positioned along the second optical waveguide605 and within an evanescent optical coupling distance of the secondoptical waveguide 605. While the example electro-optic receiver 600shows three ring resonators 609-1 to 609-3 for purposes of description,it should be understood that there is no limit on the number of thesecond plurality of ring resonators that can be positioned along thesecond optical waveguide 605, so long as the second plurality of ringresonators and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. Each of the ring resonators609-1 to 609-3 is configured to operate at a respective resonantwavelength λ₁ to λ₃, such that the polarization-rotated second portionof the incoming light having a wavelength (λ₁, λ₂, or λ₃) substantiallyequal to the respective resonant wavelength (λ₁, λ₂, or λ₃) of a givenone of the ring resonators 609-1 to 609-3 optically couples into thegiven one of the ring resonators 609-1 to 609-3. The number of thesecond plurality of ring resonators 609-1 to 609-3 is equal to thenumber of the first plurality of ring resonators 607-1 to 607-3. Also,the respective resonant wavelengths (λ₁, λ₂, λ₃) of the ring resonators609-1 to 609-3 substantially match the respective resonant wavelengths(λ₁, λ₂, λ₃) of the ring resonators 607-1 to 607-3. In some embodiments,the ring resonators 609-1 to 609-3 are implemented as annular-shapedwaveguides having circuitous configuration, e.g., circular, oval,race-track, or another arbitrary circuitous shape. In some embodiments,the ring resonators 609-1 to 609-3 are implemented as circular discs. Insome embodiments, each of the second plurality of ring resonators 609-1to 609-3 is formed to have a same shape and size as the correspondingone (with respect to resonant wavelength (λ₁, λ₂, λ₃)) of the firstplurality of ring resonators 607-1 to 607-3. The ring resonators 609-1to 609-3 are formed of a material through which light can be in-coupled,out-coupled, and guided. Each of the ring resonators 609-1 to 609-3 isformed within a surrounding material that has an optical index ofrefraction sufficiently different from that of the ring resonators 609-1to 609-3 to enable guiding of light within the ring resonators 609-1 to609-3 and around the circuitous path defined by each of the ringresonators 609-1 to 609-3. In some embodiments, each of the secondplurality of ring resonators 609-1 to 609-3 is formed of a same materialas the corresponding one (with respect to resonant wavelength (λ₁, λ₂,λ₃)) of the first plurality of ring resonators 607-1 to 607-3. In someembodiments, each of the ring resonators 609-1 to 609-3 is configured tohave an annular-shape or disc-shape with an outer diameter of less thanabout 50 micrometers. In some embodiments, each of the ring resonatorresonators 609-1 to 609-3 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 10 micrometers.

The electro-optic receiver 600 includes a first plurality of outputoptical waveguides 608-1 to 608-3 respectively positioned within anevanescent optical coupling distance of the first plurality of ringresonators 607-1 to 607-3. The electro-optic receiver 600 also includesa second plurality of output optical waveguides 610-1 to 610-3respectively positioned within an evanescent optical coupling distanceof the second plurality of ring resonators 609-1 to 609-3. Theelectro-optic receiver 600 also includes a plurality of photodetectors611-1 to 611-3. Each of the photodetectors 611-1 to 611-3 is opticallyconnected to receive light from a respective one of the first pluralityof output optical waveguides 608-1 to 608-3 and from a respective one ofthe second plurality of output optical waveguides 610-1 to 610-3, wherethe respective one of the first plurality of output optical waveguides608-1 to 608-3 is optically coupled to one of the first plurality ofring resonators 607-1 to 607-3 having a given resonant wavelength (λ₁,λ₂, or λ₃), and where the respective one of the second plurality ofoutput optical waveguides 610-1 to 610-3 is optically coupled to one ofthe second plurality of ring resonators 609-1 to 609-3 having the givenresonant wavelength (λ₁, λ₂, or λ₃). In this manner, each of thephotodetectors 611-1 to 611-3 receives incoming light of thesubstantially same wavelength from a corresponding one of the firstplurality of output optical waveguides 608-1 to 608-3 and from acorresponding one of the second plurality of output optical waveguides610-1 to 610-3.

In some embodiments, each of the photodetectors 611-1 to 611-3 isconfigured like the linear photodetector described with regard to FIG. 4, such that the corresponding one of the first plurality of outputoptical waveguides 608-1 to 608-3 is connected to one end of thephotodetector 611-1 to 611-3, and the corresponding one of the secondplurality of output optical waveguides 610-1 to 610-3 is connected tothe other end of the photodetector 611-1 to 611-3. In this manner, thephotodetector 611-1 to 611-3 is configured to absorb a majority of thefirst portion of the incoming light (having the first polarization) in afirst linear half of the photodetector 611-1 to 611-3, and absorb amajority of the polarization-rotated second portion of the incominglight (also having the first polarization) in a second linear half ofthe photodetector 611-1 to 611-3. In some embodiments, one or moreelectrical contacts (e.g., 407) positioned along the first linear halfof the photodetector 611-1 to 611-3 are electrically connected to afirst photocurrent detection circuit within the photocurrent processingcircuitry 167, and one or more electrical contacts (e.g., 407)positioned along the second linear half of the photodetector 611-1 to611-3 are electrically connected to a second photocurrent detectioncircuit within the photocurrent processing circuitry 167.

In some embodiments, the first optical waveguide 603 includes a firstsection 603A extending from the first optical output 613B of the PSR 613to a nearest one (607-1) of the first plurality of ring resonators 607-1to 607-3 to the PSR 613. Also, the second optical waveguide 605 includesa first section 605A extending from the second optical output 613C ofthe PSR 613 to a nearest one (609-1) of the second plurality of ringresonators 609-1 to 609-3 to the PSR 613. In these embodiments, eitherthe first section 603A of the first optical waveguide 603 is longer thanthe first section 605A of the second optical waveguide 605, or the firstsection 605A of the second optical waveguide 605 is longer than thefirst section 603A of the first optical waveguide 603, in order tocompensate for a timing delay between the first portion of the incominglight exiting the PSR 613 and the polarization-rotated second portion ofthe incoming light exiting the PSR 613, so as to minimize a timing-skew(timing difference) between optical coupling of the first portion of theincoming light into the first plurality of ring resonators 607-1 to607-3 and optical coupling of the polarization-rotated second portion ofthe incoming light into corresponding ones (by wavelength) of the secondplurality of ring resonators 609-1 to 609-3. In the exampleelectro-optic receiver 600, the first section 603A of the first opticalwaveguide 603 includes a delay section 603B configured so that theoptical path length through the first section 603A of the first opticalwaveguide 603 is longer than the optical path length through the firstsection 605A of the second optical waveguide 605. The delay section 603Bis configured to compensate for the timing delay between the firstportion of the incoming light exiting the PSR 613 and thepolarization-rotated second portion of the incoming light exiting thePSR 613. The length of the first section 603A of the first opticalwaveguide 603 and the length of the first section 605A of the secondoptical waveguide 605 are defined to reduce a difference in arrival timeof the first portion of the incoming light and the polarization-rotatedsecond portion of the incoming light at a closest one (611-1) of theplurality of photodetectors 611-1 to 611-3 to the PSR 613. Also, in someembodiments, the electro-optic receiver 600 includes the timing-skewmanagement system 165 to electronically compensate for a temporaldifference in photocurrent generation by a given one of the plurality ofphotodetectors 611-1 to 611-3 caused by a difference between the arrivaltime of the first portion of the incoming light at the corresponding oneof the plurality of photodetectors 607-1 to 607-3, respectively, and thearrival time of the polarization-rotated second portion of the incominglight at the corresponding one of the plurality of photodetectors 609-1to 609-3, respectively.

In some embodiments of the electro-optic receiver 600, the two opticalwaveguides (603 and 605) are separately coupled to an array of passivering filters (607-1 to 607-3 and 609-1 to 609-3, respectively). Eachpassive ring filter (607-1 to 607-3 and 609-1 to 609-3) is designed toroute the light from a single wavelength channel (λ₁, λ₂, λ₃), within anarrow wavelength range, to a corresponding output waveguide (608-1 to608-3, respectively, and 610-1 to 610-3, respectively) that is connectedto a linear photodetector (611-1 to 611-3, respectively). The lightrouting within the electro-optic receiver 600 is designed so that thetwo polarizations of incoming light in a given wavelength channel asreceived through the optical coupler 615 are routed separately, throughseparate ring filters (607-1 to 607-3 and 609-1 to 609-3), into the samelinear detector (611-1 to 611-3) from opposite directions. In someembodiments, an optical delay line (603B) may be added (for example, inthe form of a longer section of waveguide) to one of the two opticalwaveguides (603, 605) in order to compensate for asymmetric delaybetween the first portion of the incoming light and thepolarization-rotated second portion of the incoming light introduced bythe PSR 613. The electronic timing-skew management system 165 can beimplemented and operated to correct for any remaining timing-skewbetween the first portion of the incoming light and thepolarization-rotated second portion of the incoming light introduced bythe PSR 613. Also, because the first optical waveguide 603 is notoptically connected to the second optical waveguide 605, theelectro-optic receiver 600 does not require a VOA, such as previouslydescribed with regard to the VOA 163 in the electro-optic receiver 150of FIG. 1B.

In some embodiments of the electro-optic receiver 600, the photocurrentfrom a given photodetector 611-1 to 611-3 is used as a feedback signalto control the alignment between the resonance wavelength (λ₁, λ₂, λ₃)of the pair of ring resonators (607-1 to 607-3 and 609-1 to 609-3)corresponding to the given photodetector 611-1 to 611-3 and thewavelength of the corresponding data wavelength channel of the incominglight as received through the optical coupler 615. For example, in someembodiments, a control circuit is used to tune the two the resonancewavelengths of the pair of ring resonators (607-1 to 607-3 and 609-1 to609-3) corresponding to the given photodetector 611-1 to 611-3 tooptimize optical power reaching the given photodetector 611-1 to 611-3.In some embodiments, the control circuit operates to control a thermaltuner (e.g., heater) implemented to control a temperature of the pair ofring resonators (607-1 to 607-3 and 609-1 to 609-3) corresponding to thegiven photodetector 611-1 to 611-3 to optimize optical power reachingthe given photodetector 611-1 to 611-3. In some embodiments, the controlcircuit operates to control a diode built into the pair of ringresonators (607-1 to 607-3 and 609-1 to 609-3) corresponding to thegiven photodetector 611-1 to 611-3 to optimize optical power reachingthe given photodetector 611-1 to 611-3.

FIG. 7 shows a flowchart of a method for operating a photonic circuit,in accordance with some embodiments. In some embodiments, the method ofFIG. 7 is practiced using the electro-optic receiver 600. The methodincludes an operation 701 for receiving incoming light through anoptical input port (e.g., optical coupler 615) of the photonic circuit(e.g., PIC 601). A first portion of the incoming light has a firstpolarization and a second portion of the incoming light having a secondpolarization. The method also includes an operation 703 for splittingthe first portion of the incoming light from the second portion of theincoming light. In some embodiments, the operation 703 is performed bythe PSR 613. The method also includes an operation 705 for directing thefirst portion of the incoming light into a first optical waveguide(e.g., optical waveguide 603). The method also includes an operation 707for rotating the second polarization of the second portion of theincoming light to the first polarization so that the second portion ofthe incoming light is a polarization-rotated second portion of theincoming light. In some embodiments, the operation 703 is performed bythe PSR 613. The method also includes an operation 709 for directing thepolarization-rotated second portion of the incoming light into a secondoptical waveguide (e.g., optical waveguide 605). The method alsoincludes an operation 711 for operating a first plurality of ringresonators (e.g., ring resonators 607-1 to 607-3) to evanescentlyin-couple light from the first optical waveguide, where each of thefirst plurality of ring resonators is operated at a respective resonantwavelength to in-couple light having the respective resonant wavelengthfrom the first optical waveguide. The method also includes an operation713 for optically coupling light from the first plurality of ringresonators into respective ones of a first plurality of output opticalwaveguides (e.g., optical waveguides 608-1 to 608-3). The method alsoincludes an operation 715 for directing light within the first pluralityof output optical waveguides into respective ones of a plurality ofphotodetectors (e.g., photodetectors 611-1 to 611-3). The method alsoincludes an operation 717 for operating a second plurality of ringresonators (e.g., ring resonators 609-1 to 609-3) to evanescentlyin-couple light from the second optical waveguide, where each of thesecond plurality of ring resonators is operated at a respective resonantwavelength to in-couple light having the respective resonant wavelengthfrom the second optical waveguide. The method also includes an operation719 for optically coupling light from the second plurality of ringresonators into respective ones of a second plurality of output opticalwaveguides (e.g., optical waveguides 610-1 to 610-3). The method alsoincludes an operation 721 for directing light within the secondplurality of output optical waveguides into respective ones of theplurality of photodetectors.

In some embodiments, the first optical waveguide includes an inputsection (e.g., 603A), and the second optical waveguide includes an inputsection (e.g., 605A), where either the input section of the firstoptical waveguide is longer than the input section of the second opticalwaveguide, or the input section of the second optical waveguide islonger than the input section of the first optical waveguide. In theseembodiments, the method includes defining a length of the input sectionof the first optical waveguide and a length of the input section of thesecond optical waveguide to reduce a difference in arrival time of thefirst portion of the incoming light and the polarization-rotated secondportion of the incoming light at a given one of the plurality ofphotodetectors. In some embodiments, the method includes electronicallycompensating for a temporal difference in photocurrent generation by agiven one of the plurality of photodetectors caused by a difference inarrival time of the first portion of the incoming light and thepolarization-rotated second portion of the incoming light at the givenone of the plurality of photodetectors. In some embodiments, the lengthof the input section of the first optical waveguide and the length ofthe input section of the second optical waveguide are defined tocompensate for a temporal difference between directing the first portionof the incoming light into the first optical waveguide and directing thepolarization-rotated second portion of the incoming light into thesecond optical waveguide.

In some embodiments, the each of the plurality of photodetectors used inthe method is a linear photodetector (e.g., linear photodetector 400)that has a first end optically connected to a respective one of thefirst plurality of output optical waveguides and a second end opticallyconnected to a respective one of the second plurality of output opticalwaveguides. In some of these embodiments, the method includes operatingthe linear photodetector to absorb a majority of the first portion ofthe incoming light in a first half of the linear photodetector, andabsorb a majority of the polarization-rotated second portion of theincoming light in a second half of the linear photodetector. In some ofthese embodiments, the method includes operating a first photocurrentdetection circuit to detect photocurrent generated within the first halfof the linear photodetector, and operating a second photocurrentdetection circuit to detect photocurrent generated within the secondhalf of the linear photodetector. In this manner, the method providesfor determination of how much optical power is conveyed into the linearphotodetector from each of the first polarization and the secondpolarization of the original incoming optical signal. Correspondingly,in this manner, the method provides for determination of how muchoptical power was received in the original incoming optical signal ineach of the first polarization and the second polarization.

FIG. 8 shows an example configuration of an electro-optic receiver 800implemented within a PIC 801, in accordance with some embodiments. Theelectro-optic receiver 800 includes a PSR 821 that has an optical input821A optically connected to receive incoming light from an opticalcoupler 823, by way of an optical waveguide 824. In some embodiments,the optical input 821A of the PSR 821 is directly optically coupled tothe optical coupler 823, such that the optical waveguide 824 is notrequired. In some embodiments, the optical coupler 823 is implemented asan edge coupler. However, in other embodiments, the optical coupler 823is implemented as a vertical grating coupler, or as another type ofoptical coupling device that provides for optical coupling of the PIC801 to an optical fiber/waveguide 825. Incoming light is transmittedfrom the optical fiber/waveguide 825 into the optical coupler 823, asindicated by arrow 826. The PSR 821 has a first optical output 821B anda second optical output 821C. The PSR 821 is configured to direct afirst portion of the incoming light having a first polarization (TE orTM) through the first optical output 821B. The PSR 821 is alsoconfigured to rotate a polarization of a second portion of the incominglight from a second polarization (opposite of the first polarization) tothe first polarization. In this manner, the PSR 821 turns the secondportion of the incoming light into a polarization-rotated second portionof the incoming light. The PSR 821 is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output 821C. In some embodiments, the first portion ofthe incoming light having a first polarization is transmitted throughthe second optical output 821C, and the polarization-rotated secondportion of the incoming light is transmitted through the first opticaloutput 821B.

The electro-optic receiver 800 includes a first optical waveguide 803optically connected to the first optical output 821B of the PSR 821. Theelectro-optic receiver 800 also includes a second optical waveguide 805optically connected to the second optical output 821C of the PSR 821.The first optical waveguide 803 and the second optical waveguide 805 areformed of a material through which light can be in-coupled, out-coupled,and guided. Each of the first optical waveguide 803 and the secondoptical waveguide 805 is formed within a surrounding material that hasan optical index of refraction sufficiently different from that of thefirst optical waveguide 803 and the second optical waveguide 805,respectively, to enable guiding of light within the first opticalwaveguide 803 and the second optical waveguide 805. In some embodiments,first optical waveguide 803 and the second optical waveguide 805 areformed of a same material. In some embodiments, the first portion of theincoming light is transmitted through the first optical output 821B ofthe PSR 821 and into the first optical waveguide 803, and travels alongthe first optical waveguide 803, as indicated by arrow 804. Also, inthese embodiments, the polarization-rotated second portion of theincoming light is transmitted through the second optical output 821C ofthe PSR 821 and into the second optical waveguide 805, and travels alongthe second optical waveguide 805, as indicated by arrow 806.Alternatively, in some embodiments, the first portion of the incominglight is transmitted through the second optical output 821C of the PSR821 and into the second optical waveguide 805, and travels along thesecond optical waveguide 805, as indicated by arrow 806. Also, in thesealternative embodiments, the polarization-rotated second portion of theincoming light is transmitted through the first optical output 821B ofthe PSR 821 and into the first optical waveguide 803, and travels alongthe first optical waveguide 803, as indicated by arrow 804.

The electro-optic receiver 800 also includes a two-by-two opticalsplitter 809 that has a first optical input 809A optically connected tothe second end of the first optical waveguide 803. The two-by-twooptical splitter 809 has a second optical input 809B optically connectedto the second end of the second optical waveguide 805. The two-by-twooptical splitter 809 has a first optical output 809C and a secondoptical output 809D. The two-by-two optical splitter 809 is configuredto output some of the first portion of the incoming light and some ofthe polarization-rotated second portion of the incoming light througheach of the first optical output 809C and the second optical output 809Dof the two-by-two optical splitter 809. In some embodiments, thetwo-by-two optical splitter 809 is an even 50-50 optical splitter.However, in other embodiments, the two-by-two optical splitter 809 isnot an even 50-50 optical splitter. The optical splitting ratio of thetwo-by-two optical splitter 809 defines how much optical power istransmitted to each of the first optical output 809C and the secondoptical output 809D from each of the first optical input 809A and thesecond optical input 809B. The optical splitting ratio provided by thetwo-by-two optical splitter 809 is set and/or controlled to ensure thatvery low optical power transmission through either the first opticaloutput 809C or the second optical output 809D is avoided for any of thewavelength channels of the incoming light received through the firstoptical input 809A and the second optical input 809B. Also, in someembodiments, the two-by-two optical splitter 809 is a non-broadbandoptical splitter.

In some embodiments, a phase shifter 807 is optically coupled to eitherthe first optical waveguide 803 or the second optical waveguide 805. Theexample electro-optic receiver 800 has the phase shifter 807 opticallycoupled to the second optical waveguide 805. In some embodiments, thephase shifter 807 is implemented as a thermal tuner (e.g., heatingdevice) positioned over the second optical waveguide 805, which operatesby exploiting the thermo-optic effect of the second optical waveguide805 material. In some embodiments, the phase shifter 807 is implementedas an electro-optic device (e.g., diode) built into the second opticalwaveguide 805, which operates by exploiting electro-optic effects withinthe second optical waveguide 805. In some embodiments, the phase shifter807 is implemented as one or more ring resonator phase shifters.

In these embodiments, either the first optical waveguide 803 is longerthan the second optical waveguide 805, or the second optical waveguide805 is longer than the first optical waveguide 803, in order tocompensate for a timing delay between the first portion of the incominglight exiting the PSR 821 and the polarization-rotated second portion ofthe incoming light exiting the PSR 821, so as to minimize a timing-skew(timing difference) between arrival of the first portion of the incominglight into the two-by-two optical splitter 809 and arrival of thepolarization-rotated second portion of the incoming light into thetwo-by-two optical splitter 809. In the example electro-optic receiver800, the first optical waveguide 803 includes a delay section 803Aconfigured so that the optical path length through the first opticalwaveguide 803 is longer than the optical path length through the secondoptical waveguide 805. The delay section 803A is configured tocompensate for the timing delay between the first portion of theincoming light exiting the PSR 821 and the polarization-rotated secondportion of the incoming light exiting the PSR 821. In some embodiments,the phase shifter 807 is optically coupled to a shorter one of the firstoptical waveguide 803 and the second optical waveguide 805.

In some embodiments, the delay section 803A is defined tocompensate/minimize the timing-skew between arrival of the first portionof the incoming light and the polarization-rotated second portion of theincoming light at the two-by-two optical splitter 809 when theelectro-optic receiver 800 is implemented to operate over a broad rangeof optical wavelengths, rather than just at a single optical wavelength.If a group delay difference between the two polarizations is notsufficiently compensated/minimized, a phase difference between the firstportion of the incoming light in the first optical waveguide 803 and thepolarization-rotated second portion of the incoming light in the secondoptical waveguide 805 will depend on the wavelength of the light, suchthat the single phase shifter 807 may not be able to set an appropriatephase for all wavelengths of interest. The delay section 803A is definedto ensure that the group delay difference between the two polarizationsis sufficiently compensated/minimized so that the phase differencebetween the first portion of the incoming light in the first opticalwaveguide 803 and the polarization-rotated second portion of theincoming light in the second optical waveguide 805 does not vary as afunction of the wavelength of the light, which allows the single phaseshifter 807 to set an appropriate phase for all channel wavelengths ofinterest within the incoming optical signal received through the opticalcoupler 823. The combination of the PSR 821, the first optical waveguide803 with the delay section 803A, the second optical waveguide 805, thetwo-by-two optical splitter 809, the phase shifter 807 constitutes apolarization equalizer 812.

The electro-optic receiver 800 includes a third optical waveguide 811optically connected to the first optical output 809C of the two-by-twooptical splitter 809. The electro-optic receiver 800 also includes afourth optical waveguide 813 optically connected to the second opticaloutput 809D of the two-by-two optical splitter 809. In the electro-opticreceiver 800, the third optical waveguide 811 and the fourth opticalwaveguide 813 are not optically connected/coupled to each other. Thethird optical waveguide 811 and the fourth optical waveguide 813 areformed of a material through which light can be in-coupled, out-coupled,and guided. Each of the third optical waveguide 811 and the fourthoptical waveguide 813 is formed within a surrounding material that hasan optical index of refraction sufficiently different from that of thethird optical waveguide 811 and the fourth optical waveguide 813,respectively, to enable guiding of light within the third opticalwaveguide 811 and the fourth optical waveguide 813. In some embodiments,third optical waveguide 811 and the fourth optical waveguide 813 areformed of a same material. Some of the first portion of the incominglight (having the first polarization) is directed/conveyed through thefirst optical output 809C of the two-by-two optical splitter 809 andinto a third optical waveguide 811. Also, some of the first portion ofthe incoming light (having the first polarization) is directed/conveyedthrough the second optical output 809D of the two-by-two opticalsplitter 809 and into a fourth optical waveguide 813. Some of thepolarization-rotated second portion of the incoming light (having thefirst polarization) is directed/conveyed through the first opticaloutput 809C of the two-by-two optical splitter 809 and into the thirdoptical waveguide 811. Also, some of the polarization-rotated secondportion of the incoming light (having the first polarization) isdirected/conveyed through the second optical output 809D of thetwo-by-two optical splitter 809 and into the fourth optical waveguide813.

The electro-optic receiver 800 includes a first plurality of ringresonators 815-1 to 815-3 positioned along the third optical waveguide811 and within an evanescent optical coupling distance of the thirdoptical waveguide 811. While the example electro-optic receiver 800shows three ring resonators 815-1 to 815-3 for purposes of description,it should be understood that there is no limit on the number of thefirst plurality of ring resonators that can be positioned along thethird optical waveguide 811, so long as the first plurality of ringresonators and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. Each of the ring resonators815-1 to 815-3 is configured to operate at a respective resonantwavelength λ₁ to λ₃, such that the first portion of the incoming lightand the polarization-rotated second portion of the incoming light havinga wavelength (λ₁, λ₂, or λ₃) substantially equal to the respectiveresonant wavelength (λ₁, λ₂, or λ₃) of a given one of the ringresonators 815-1 to 815-3 optically couples into the given one of thering resonators 815-1 to 815-3 from the third optical waveguide 811. Insome embodiments, the ring resonators 815-1 to 815-3 are implemented asannular-shaped waveguides having circuitous configuration, e.g.,circular, oval, race-track, or another arbitrary circuitous shape. Insome embodiments, the ring resonators 815-1 to 815-3 are implemented ascircular discs. The ring resonators 815-1 to 815-3 are formed of amaterial through which light can be in-coupled, out-coupled, and guided.Each of the ring resonators 815-1 to 815-3 is formed within asurrounding material that has an optical index of refractionsufficiently different from that of the ring resonators 815-1 to 815-3to enable guiding of light within the ring resonators 815-1 to 815-3 andaround the circuitous path defined by each of the ring resonators 815-1to 815-3. In some embodiments, each of the ring resonators 815-1 to815-3 is configured to have an annular-shape or disc-shape with an outerdiameter of less than about 50 micrometers. In some embodiments, each ofthe ring resonators 815-1 to 815-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 10micrometers.

The electro-optic receiver 800 includes a second plurality of ringresonators 817-1 to 817-3 positioned along the fourth optical waveguide813 and within an evanescent optical coupling distance of the fourthoptical waveguide 813. While the example electro-optic receiver 800shows three ring resonators 817-1 to 817-3 for purposes of description,it should be understood that there is no limit on the number of thesecond plurality of ring resonators that can be positioned along thesecond optical waveguide 813, so long as the second plurality of ringresonators and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. Each of the ring resonators817-1 to 817-3 is configured to operate at a respective resonantwavelength λ₁ to λ₃, such that the polarization-rotated second portionof the incoming light having a wavelength (λ₁, λ₂, or λ₃) substantiallyequal to the respective resonant wavelength (λ₁, λ₂, or λ₃) of a givenone of the ring resonators 609-1 to 609-3 optically couples into thegiven one of the ring resonators 817-1 to 817-3 from the fourth opticalwaveguide 813. The number of the second plurality of ring resonators817-1 to 817-3 is equal to the number of the first plurality of ringresonators 815-1 to 815-3. Also, the respective resonant wavelengths(λ₁, λ₂, λ₃) of the ring resonators 817-1 to 817-3 substantially matchthe respective resonant wavelengths (λ₁, λ₂, λ₃) of the ring resonators815-1 to 815-3. In some embodiments, the ring resonators 817-1 to 817-3are implemented as annular-shaped waveguides having circuitousconfiguration, e.g., circular, oval, race-track, or another arbitrarycircuitous shape. In some embodiments, the ring resonators 817-1 to817-3 are implemented as circular discs. In some embodiments, each ofthe second plurality of ring resonators 817-1 to 817-3 is formed to havea same shape and size as the corresponding one (with respect to resonantwavelength (λ₁, λ₂, λ₃)) of the first plurality of ring resonators 815-1to 815-3. The ring resonators 817-1 to 817-3 are formed of a materialthrough which light can be in-coupled, out-coupled, and guided. Each ofthe ring resonators 817-1 to 817-3 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the ring resonators 817-1 to 817-3 to enable guiding oflight within the ring resonators 817-1 to 817-3 and around thecircuitous path defined by each of the ring resonators 817-1 to 817-3.In some embodiments, each of the second plurality of ring resonators817-1 to 817-3 is formed of a same material as the corresponding one(with respect to resonant wavelength (λ₁, λ₂, λ₃)) of the firstplurality of ring resonators 815-1 to 815-3. In some embodiments, eachof the ring resonators 817-1 to 817-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonators 817-1 to817-3 is configured to have an annular-shape or disc-shape with an outerdiameter of less than about 10 micrometers.

The electro-optic receiver 800 includes a first plurality of outputoptical waveguides 816-1 to 816-3 respectively positioned within anevanescent optical coupling distance of the first plurality of ringresonators 815-1 to 815-3. The electro-optic receiver 800 also includesa second plurality of output optical waveguides 818-1 to 818-3respectively positioned within an evanescent optical coupling distanceof the second plurality of ring resonators 817-1 to 817-3. Theelectro-optic receiver 800 also includes a plurality of photodetectors819-1 to 819-3. Each of the photodetectors 819-1 to 819-3 is opticallyconnected to receive light from a respective one of the first pluralityof output optical waveguides 816-1 to 816-3 and from a respective one ofthe second plurality of output optical waveguides 818-1 to 818-3, wherethe respective one of the first plurality of output optical waveguides816-1 to 816-3 is optically coupled to one of the first plurality ofring resonators 815-1 to 815-3 having a given resonant wavelength (λ₁,λ₂, or λ₃), and where the respective one of the second plurality ofoutput optical waveguides 818-1 to 818-3 is optically coupled to one ofthe second plurality of ring resonators 817-1 to 817-3 having the samegiven resonant wavelength (λ₁, λ₂, or λ₃). In this manner, each of thephotodetectors 819-1 to 819-3 receives incoming light of thesubstantially same wavelength from a corresponding one of the firstplurality of output optical waveguides 816-1 to 816-3 and from acorresponding one of the second plurality of output optical waveguides818-1 to 818-3.

In some embodiments, each of the photodetectors 819-1 to 819-3 isconfigured like the linear photodetector described with regard to FIG. 4, such that the corresponding one of the first plurality of outputoptical waveguides 816-1 to 816-3 is connected to one end of thephotodetector 819-1 to 819-3, and the corresponding one of the secondplurality of output optical waveguides 818-1 to 818-3 is connected tothe other end of the photodetector 819-1 to 819-3. In this manner, thephotodetector 819-1 to 819-3 is configured to absorb a majority of thefirst portion of the incoming light (having the first polarization) in afirst linear half of the photodetector 819-1 to 819-3, and absorb amajority of the polarization-rotated second portion of the incominglight (also having the first polarization) in a second linear half ofthe photodetector 819-1 to 819-3. In some embodiments, one or moreelectrical contacts (e.g., 407) positioned along the first linear halfof the photodetector 819-1 to 819-3 are electrically connected to afirst photocurrent detection circuit within the photocurrent processingcircuitry 167, and one or more electrical contacts (e.g., 407)positioned along the second linear half of the photodetector 819-1 to819-3 are electrically connected to a second photocurrent detectioncircuit within the photocurrent processing circuitry 167. Also, in someembodiments, the electro-optic receiver 800 includes the timing-skewmanagement system 165 to electronically compensate for a temporaldifference in photocurrent generation by a given one of the plurality ofphotodetectors 819-1 to 819-3 caused by a difference between the arrivaltime of the first portion of the incoming light at the corresponding oneof the plurality of photodetectors 819-1 to 819-3, respectively, and thearrival time of the polarization-rotated second portion of the incominglight at the corresponding one of the plurality of photodetectors 819-1to 819-3, respectively.

The electro-optic receiver 800 addresses a possible problematicsituation in which either the first optical waveguide 803 or the secondoptical waveguide 805 conveys very little light due to most or all ofthe incoming light, as received through the optical coupler 823, havingone polarization (either mostly TE or mostly TM). In this situation, ifthe two-by-two optical splitter 809 were not implemented, it would bevery difficult for any ring tuning algorithm to keep the operatingresonant wavelengths of the ring resonators 815-1 to 815-3 and 817-1 to817-3 aligned with the corresponding channel wavelengths, respectively,in the incoming light signal, as received through the optical coupler823. Also, the above-mentioned situation is even more problematic whenthe polarization in the optical fiber/waveguide 825 evolves over time,because the ring resonators 815-1 to 815-3 and 817-1 to 817-3 will haveto re-lock to the channel wavelengths as the optical power ramps up. Ifthe ring resonators 815-1 to 815-3 and 817-1 to 817-3 have to re-lock tochanging channel wavelengths, an interruption will occur in the datasignal output by the electro-optic receiver 800. To address theabove-mentioned situation, the electro-optic receiver 800 implements thetwo-by-two optical splitter 809 and the phase shifter 807 to ensurenon-negligible optical power in each of the third optical waveguide 811and the fourth optical waveguide 813 before the light reaches the ringresonators 815-1 to 815-3 and 817-1 to 817-3. The two-by-two opticalsplitter 809 ensures that each of the third optical waveguide 811 andthe fourth optical waveguide 813 conveys enough light of the firstpolarization to ensure that the ring resonators 815-1 to 815-3 and 817-1to 817-3 can lock onto and maintain respective resonant wavelengths thatsubstantially align with the channel wavelengths in the incoming lightsignal. In some embodiments, the phase shifter 807 uses active controlas the polarization in the optical fiber/waveguide 825 drifts over time.The active control of the phase shifter 807 is implemented by activecontrol circuitry (feedback circuitry 1015). For example, in someembodiments, active control of the phase shifter 807 is implemented byactive control circuitry that measures optical power in the ringresonators 815-1 to 815-3 and 817-1 to 817-3, and uses that measuredoptical power as feedback signals to adjust the operation of the phaseshifter 807 as needed to track with the polarization in the opticalfiber/waveguide 825.

FIG. 9 shows a flowchart of a method for operating a photonic integratedcircuit, in accordance with some embodiments. In some embodiments, themethod of FIG. 9 is practiced using the electro-optic receiver 800. Themethod includes an operation 901 for receiving incoming light through anoptical input port (e.g., optical coupler 823) of the photonic circuit(e.g., PIC 801). A first portion of the incoming light has a firstpolarization and a second portion of the incoming light has a secondpolarization. The method also includes an operation 903 for splittingthe first portion of the incoming light from the second portion of theincoming light. In some embodiments, the operation 903 is performed bythe PSR 821. The method also includes an operation 905 for directing thefirst portion of the incoming light through a first optical waveguide(e.g., optical waveguide 803) and into a first optical input (e.g.,809A) of a two-by-two splitter (e.g., 809). In some embodiments, theoperation 905 is performed by the PSR 821. The method also includes anoperation 907 for rotating the second polarization of the second portionof the incoming light to the first polarization so that the secondportion of the incoming light is a polarization-rotated second portionof the incoming light. In some embodiments, the operation 907 isperformed by the PSR 821. The method also includes an operation 909 fordirecting the polarization-rotated second portion of the incoming lightthrough a second optical waveguide (e.g., optical waveguide 805) andinto a second optical input (e.g., 809B) of the two-by-two splitter. Insome embodiments, the operation 909 is performed by the PSR 821. Themethod also includes an operation 911 for directing some of the firstportion of the incoming light through a first optical output (e.g.,809C) of the two-by-two optical splitter and into a third opticalwaveguide (e.g., optical waveguide 811). The method also includes anoperation 913 for directing some of the first portion of the incominglight through a second optical output (e.g., 809D) of the two-by-twooptical splitter and into a fourth optical waveguide (e.g., opticalwaveguide 813). The method also includes an operation 915 for directingsome of the polarization-rotated second portion of the incoming lightthrough the first optical output of the two-by-two optical splitter andinto the third optical waveguide. The method also includes an operation917 for directing some of the polarization-rotated second portion of theincoming light through the second optical output of the two-by-twooptical splitter and into the fourth optical waveguide. In someembodiments, the operations 911 through 917 are performed by thetwo-by-two optical splitter 809.

The method of FIG. 9 also includes an operation 919 for operating afirst plurality of ring resonators (e.g., ring resonators 815-1 to815-3) to evanescently in-couple light from the third optical waveguide,where each of the first plurality of ring resonators is operated at arespective resonant wavelength to in-couple light having the respectiveresonant wavelength from the third optical waveguide. The method alsoincludes an operation 921 for optically coupling light from the firstplurality of ring resonators into respective ones of a first pluralityof output optical waveguides (e.g., optical waveguides 816-1 to 816-3).The method also includes an operation 923 for directing light within thefirst plurality of output optical waveguides into respective ones of aplurality of photodetectors (e.g., photodetectors 819-1 to 819-3). Themethod also includes an operation 925 for operating a second pluralityof ring resonators (e.g., ring resonators 817-1 to 817-3) toevanescently in-couple light from the fourth optical waveguide, whereeach of the second plurality of ring resonators is operated at arespective resonant wavelength to in-couple light having the respectiveresonant wavelength from the fourth optical waveguide. The method alsoincludes an operation 927 for optically coupling light from the secondplurality of ring resonators into respective ones of a second pluralityof output optical waveguides (e.g., optical waveguides 818-1 to 818-3).The method also includes an operation 929 for directing light within thesecond plurality of output optical waveguides into respective ones ofthe plurality of photodetectors.

In some embodiments, the method of FIG. 9 also includes operating aphase shifter in optical coupling with either the first opticalwaveguide or the second optical waveguide to apply a controlled amountof shift to a phase of light traveling through either the first opticalwaveguide or the second optical waveguide to which the phase shifter isoptically coupled. In some embodiments, the first optical waveguide islonger than the second optical waveguide, or the second opticalwaveguide is longer than the first optical waveguide. In theseembodiments, the method of FIG. 9 includes defining a length of thefirst optical waveguide and a length of the second optical waveguide toreduce a difference in arrival time of the first portion of the incominglight and the polarization-rotated second portion of the incoming lightat the two-by-two optical splitter. In some embodiments, the methodincludes electronically compensating for a temporal difference inphotocurrent generation by a given one of the plurality ofphotodetectors caused by a difference in arrival time of the firstportion of the incoming light and the polarization-rotated secondportion of the incoming light at the given one of the plurality ofphotodetectors. In some embodiments, the length of the first opticalwaveguide and the length of the second optical waveguide are defined tocompensate for a temporal difference between directing the first portionof the incoming light into the first optical waveguide and directing thepolarization-rotated second portion of the incoming light into thesecond optical waveguide.

In some embodiments, the each of the plurality of photodetectors used inthe method of FIG. 9 is a linear photodetector (e.g., linearphotodetector 400) that has a first end optically connected to arespective one of the first plurality of output optical waveguides and asecond end optically connected to a respective one of the secondplurality of output optical waveguides. In some of these embodiments,the method of FIG. 9 includes operating the linear photodetector toabsorb a majority of the first portion of the incoming light in a firsthalf of the linear photodetector, and absorb a majority of thepolarization-rotated second portion of the incoming light in a secondhalf of the linear photodetector. In some of these embodiments, themethod of FIG. 9 includes operating a first photocurrent detectioncircuit to detect photocurrent generated within the first half of thelinear photodetector, and operating a second photocurrent detectioncircuit to detect photocurrent generated within the second half of thelinear photodetector. In this manner, the method of FIG. 9 provides fordetermination of how much optical power is conveyed into the linearphotodetector from each of the first polarization and the secondpolarization of the original incoming optical signal. Correspondingly,in this manner, the method provides for determination of how muchoptical power was received in the incoming optical signal in each of thefirst polarization and the second polarization.

FIG. 10A shows an example configuration of an optical input polarizationmanagement device 1000 implemented within a PIC 1001, in accordance withsome embodiments. The optical input polarization management device 1000includes a polarization controller 1003 that has an optical input 1003Aoptically connected to receive incoming light from an optical coupler1005, by way of an optical waveguide 1006. In some embodiments, theoptical input 1003A of the polarization controller 1003 is directlyoptically coupled to the optical coupler 1005, such that the opticalwaveguide 1006 is not required. In some embodiments, the optical coupler1005 is implemented as an edge coupler. However, in other embodiments,the optical coupler 1005 is implemented as a vertical grating coupler,or as another type of optical coupling device that provides for opticalcoupling of the polarization controller 1003 to an opticalfiber/waveguide 1007. Incoming light is transmitted from the opticalfiber/waveguide 1007 into the optical coupler 1005, as indicated byarrow 1008. The optical fiber/waveguide 1007 is optically connected toreceive and convey light from a multi-wavelength light source 1009. Insome embodiments, the multi-wavelength light source 1009 is configuredto transmit multiple wavelengths of continuous wave laser light throughthe optical fiber/waveguide 1007. In some embodiments, a polarization ofthe light transmitted by the multi-wavelength light source 1009 throughthe optical fiber/waveguide 1007 is uncontrolled and possibly variesover time.

The on-chip polarization controller 1003 is configured to combine thetwo polarizations of the incoming light as received through the opticalinput 1003A as a single polarization of light and output the singlepolarization of light through an optical output 1003B of thepolarization controller 1003 in a low loss manner. For example, in someembodiments, the polarization controller 1003 is configured to receiveboth TE and TM polarizations of light through the optical input 1003A,rotate the TE polarized light to TM polarized light, and transmitessentially all of the light received through the optical input 1003A asTM polarized light through the optical output 1003B. Conversely, in someembodiments, the polarization controller 1003 is configured to receiveboth TE and TM polarizations of light through the optical input 1003A,rotate the TM polarized light to TE polarized light, and transmitessentially all of the light received through the optical input 1003A asTE polarized light through the optical output 1003B. In someembodiments, the polarization controller 1003 is electronically tunableto accommodate a power difference and a phase difference between the twopolarizations (TE and TM) within the incoming light that are unknown andpossibly varying with time.

The optical input polarization management device 1000 includes an outputoptical waveguide 1011 optically connected to the optical output of thepolarization controller 1003. In some embodiments, feedback circuitry1015 is configured to control the polarization controller 1003 based onthe light transmitted through the optical output waveguide 1011. A smallfraction of the optical power in the output waveguide 1011 is opticallytapped and measured to serve as an input signal to the feedbackcircuitry 1015. In some embodiments, a directional optical coupler isimplemented as an optical tap to transfer a small fraction of theoptical power in the optical waveguide 1011 to a tap-off waveguide 1017,which is then incident on a photodetector 1019, e.g., linearphotodetector. In some embodiments, the photodetector 1019 is configuredto detect a fraction of the optical power in all wavelength channels. Insome embodiments, a series of ring resonator filters 1013-1 to 1013-1are designed to tap a small fraction of optical power from a singlewavelength channel in the output waveguide 1011 and detect it, either bya photodetector placed within the ring resonator filter 1013-1 to 1013-3itself, or by sending the optical signal to an output waveguideconnected to a linear detector, such as previously described with regardto the ring resonators 815-1 to 815-3, output optical waveguides 816-1to 816-3, and photodetectors 819-1 to 819-3 in the electro-opticreceiver 800. In this embodiment, the optical power in each wavelengthchannel (λ₁, λ₂, λ₃) can be measured separately, which enables thefeedback circuitry 1015 to separately and independently optimize theoperation of the polarization controller 1003 for each wavelengthchannel in the incoming optical signal as received through the opticalinput 1003A of the polarization controller 1003.

While the example optical input polarization management device 1000shows three ring resonator filters 1013-1 to 1013-3 for purposes ofdescription, it should be understood that there is no limit on thenumber of the ring resonator filters that can be positioned along theoutput waveguide 1011, so long as the ring resonator filters 1013-1 to1013-3 and associated signal processing circuitry can be spatially andelectrically accommodated on the chip. Each of the ring resonatorfilters 1013-1 to 1013-3 is configured to operate at a respectiveresonant wavelength λ₁ to λ₃, such that light within the outputwaveguide 1011 having a wavelength (λ₁, λ₂, or λ₃) substantially equalto the respective resonant wavelength (λ₁, λ₂, or λ₃) of a given one ofthe ring resonator filters 1013-1 to 1013-3 optically couples into thegiven one of the ring resonator filters 1013-1 to 1013-3 from the outputwaveguide 1011. In some embodiments, the ring resonator filters 1013-1to 1013-3 are implemented as annular-shaped waveguides having circuitousconfiguration, e.g., circular, oval, race-track, or another arbitrarycircuitous shape. In some embodiments, the ring resonator filters 1013-1to 1013-3 are implemented as circular discs. The ring resonator filters1013-1 to 1013-3 are formed of a material through which light can bein-coupled, out-coupled, and guided. Each of the ring resonator filters1013-1 to 1013-3 is formed within a surrounding material that has anoptical index of refraction sufficiently different from that of the ringresonator filters 1013-1 to 1013-3 to enable guiding of light within thering resonator filters 1013-1 to 1013-3 and around the circuitous pathdefined by each of the ring resonator filters 1013-1 to 1013-3. In someembodiments, each of the ring resonator filters 1013-1 to 1013-3 isconfigured to have an annular-shape or disc-shape with an outer diameterof less than about 50 micrometers. In some embodiments, each of the ringresonator filters 1013-1 to 1013-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 10micrometers.

FIG. 10B shows the optical input polarization management device 1000 ofFIG. 10A, with an example implementation of the polarization controller1003, in accordance with some embodiments. The polarization controller1003 includes a PSR 1021 that has an optical input 1021A opticallyconnected to receive incoming light from the optical input 1003A of thepolarization controller 1003. In some embodiments, the optical input1021A of the PSR 1021 is the optical input 1003A of the polarizationcontroller 1003. The PSR 1021 has a first optical output 1021B and asecond optical output 1021C. The PSR 1021 is configured to direct afirst portion of the incoming light having a first polarization (TE orTM) through the first optical output 1021B. The PSR 1021 is alsoconfigured to rotate a polarization of a second portion of the incominglight from a second polarization (opposite of the first polarization) tothe first polarization. In this manner, the PSR 1021 turns the secondportion of the incoming light into a polarization-rotated second portionof the incoming light. The PSR 1021 is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output 1021C. In some alternative embodiments, the firstportion of the incoming light having a first polarization is transmittedthrough the second optical output 1021C, and the polarization-rotatedsecond portion of the incoming light is transmitted through the firstoptical output 1021B.

The polarization controller 1003 includes a first optical waveguide 1023optically connected to the first optical output 1021B of the PSR 1021.The polarization controller 1021 also includes a second opticalwaveguide 1025 optically connected to the second optical output 1021C ofthe PSR 1021. The first optical waveguide 1023 and the second opticalwaveguide 1025 are formed of a material through which light can bein-coupled, out-coupled, and guided. Each of the first optical waveguide1023 and the second optical waveguide 1025 is formed within asurrounding material that has an optical index of refractionsufficiently different from that of the first optical waveguide 1023 andthe second optical waveguide 1025, respectively, to enable guiding oflight within the first optical waveguide 1023 and the second opticalwaveguide 1025. In some embodiments, first optical waveguide 1023 andthe second optical waveguide 1025 are formed of a same material. In someembodiments, the first portion of the incoming light is transmittedthrough the first optical output 1021B of the PSR 1021 and into thefirst optical waveguide 1023, and travels along the first opticalwaveguide 1023, as indicated by arrow 1024. Also, in these embodiments,the polarization-rotated second portion of the incoming light istransmitted through the second optical output 1021C of the PSR 1021 andinto the second optical waveguide 1025, and travels along the secondoptical waveguide 1025, as indicated by arrow 1026. Conversely, in somealternative embodiments, the first portion of the incoming light istransmitted through the second optical output 1021C of the PSR 1021 andinto the second optical waveguide 1025, and travels along the secondoptical waveguide 1025, as indicated by arrow 1026. Also, in thesealternative embodiments, the polarization-rotated second portion of theincoming light is transmitted through the first optical output 1021B ofthe PSR 1021 and into the first optical waveguide 1023, and travelsalong the first optical waveguide 1023, as indicated by arrow 1024.

The polarization controller 1003 also includes a first two-by-twooptical splitter 1029 that has a first optical input 1029A opticallyconnected to the second end of the first optical waveguide 1023. Thefirst two-by-two optical splitter 1029 has a second optical input 1029Boptically connected to the second end of the second optical waveguide1025. The first two-by-two optical splitter 1029 has a first opticaloutput 1029C and a second optical output 1029D. The first two-by-twooptical splitter 1029 is configured to output some of the first portionof the incoming light and some of the polarization-rotated secondportion of the incoming light through each of the first optical output1029C and the second optical output 1029D of the first two-by-twooptical splitter 1029. In some embodiments, the first two-by-two opticalsplitter 1029 is an even 50-50 optical splitter. However, in otherembodiments, the first two-by-two optical splitter 1029 is not an even50-50 optical splitter. The optical splitting ratio of the firsttwo-by-two optical splitter 1029 defines how much optical power istransmitted to each of the first optical output 1029C and the secondoptical output 1029D from each of the first optical input 1029A and thesecond optical input 1029B. The optical splitting ratio provided by thefirst two-by-two optical splitter 1029 is set and/or controlled toensure that very low optical power transmission through either the firstoptical output 1029C or the second optical output 1029D is avoided forany of the wavelength channels of the incoming light received throughthe first optical input 1029A and the second optical input 1029B. Also,in some embodiments, the first two-by-two optical splitter 1029 is anon-broadband optical splitter. In some embodiments, the firsttwo-by-two optical splitter 1029 is implemented using a multi-modeinterference device (MMI) or a directional waveguide coupler, e.g., anadiabatic directional coupler.

In some embodiments, a first phase shifter 1027 is optically coupled toeither the first optical waveguide 1023 or the second optical waveguide1025. The example polarization controller 1003 has the first phaseshifter 1027 optically coupled to the second optical waveguide 1025. Insome embodiments, the first phase shifter 1027 is implemented as athermal tuner (e.g., heating device) positioned over the second opticalwaveguide 1025, which operates by exploiting the thermo-optic effect ofthe second optical waveguide 1025 material. In some embodiments, thefirst phase shifter 1027 is implemented as an electro-optic device(e.g., diode) built into the second optical waveguide 1025, whichoperates by exploiting electro-optic effects within the second opticalwaveguide 1025. In some embodiments, the first phase shifter 1027 isimplemented as one or more ring resonators, in which each of these ringresonators operates at a particular wavelength to shift the phase oflight at the particular wavelength within the optical waveguide to whichthe first phase shifter 1027 is optically coupled.

In these embodiments, either the first optical waveguide 1023 is longerthan the second optical waveguide 1025, or the second optical waveguide1025 is longer than the first optical waveguide 1023, in order tocompensate for a timing delay between the first portion of the incominglight exiting the PSR 1021 and the polarization-rotated second portionof the incoming light exiting the PSR 1021, so as to minimize atiming-skew (timing difference) between arrival of the first portion ofthe incoming light into the first two-by-two optical splitter 1029 andarrival of the polarization-rotated second portion of the incoming lightinto the first two-by-two optical splitter 1029. In the examplepolarization controller 1003, the first optical waveguide 1023 includesa delay section 1023A configured so that the optical path length throughthe first optical waveguide 1023 is longer than the optical path lengththrough the second optical waveguide 1025. The delay section 1023A isconfigured to compensate for the timing delay between the first portionof the incoming light exiting the PSR 1021 and the polarization-rotatedsecond portion of the incoming light exiting the PSR 1021. In someembodiments, the phase shifter 1027 is optically coupled to a shorterone of the first optical waveguide 1023 and the second optical waveguide1025.

In some embodiments, the delay section 1023A is defined tocompensate/minimize the timing-skew between arrival of the first portionof the incoming light and the polarization-rotated second portion of theincoming light at the first two-by-two optical splitter 1029 when thepolarization controller 1003 is implemented to operate over a broadrange of optical wavelengths, rather than just at a single opticalwavelength. If a group delay difference between the two polarizations isnot sufficiently compensated/minimized, a phase difference between thefirst portion of the incoming light in the first optical waveguide 1023and the polarization-rotated second portion of the incoming light in thesecond optical waveguide 1025 will depend on the wavelength of thelight, such that the single phase shifter 1027 may not be able to set anappropriate phase for all wavelengths of interest. The delay section1023A is defined to ensure that the group delay difference between thetwo polarizations is sufficiently compensated/minimized so that thephase difference between the first portion of the incoming light in thefirst optical waveguide 1023 and the polarization-rotated second portionof the incoming light in the second optical waveguide 1025 does not varyas a function of the wavelength of the light, which allows the phaseshifter 1027 to set an appropriate phase for all channel wavelengths ofinterest within the incoming optical signal received through the opticalcoupler 1005.

The polarization controller 1003 includes a third optical waveguide 1031optically connected to the first optical output 1029C of the firsttwo-by-two optical splitter 1029. The polarization controller 1003 alsoincludes a fourth optical waveguide 1033 optically connected to thesecond optical output 1029D of the first two-by-two optical splitter1029. The third optical waveguide 1031 and the fourth optical waveguide1033 are formed of a material through which light can be in-coupled,out-coupled, and guided. Each of the third optical waveguide 1031 andthe fourth optical waveguide 1033 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the third optical waveguide 1031 and the fourth opticalwaveguide 1033, respectively, to enable guiding of light within thethird optical waveguide 1031 and the fourth optical waveguide 1033. Insome embodiments, third optical waveguide 1031 and the fourth opticalwaveguide 1033 are formed of a same material. Some of the first portionof the incoming light (having the first polarization) isdirected/conveyed through the first optical output 1029C of the firsttwo-by-two optical splitter 1029 and into a third optical waveguide1031. Also, some of the first portion of the incoming light (having thefirst polarization) is directed/conveyed through the second opticaloutput 1029D of the first two-by-two optical splitter 1029 and into afourth optical waveguide 1033. Some of the polarization-rotated secondportion of the incoming light (having the first polarization) isdirected/conveyed through the first optical output 1029C of the firsttwo-by-two optical splitter 1029 and into the third optical waveguide1031. Also, some of the polarization-rotated second portion of theincoming light (having the first polarization) is directed/conveyedthrough the second optical output 1029D of the first two-by-two opticalsplitter 1029 and into the fourth optical waveguide 1033.

The polarization controller 1003 also includes a second two-by-twooptical splitter 1037 that has a first optical input 1037A opticallyconnected to the second end of the third optical waveguide 1031. Thesecond two-by-two optical splitter 1037 has a second optical input 1037Boptically connected to the second end of the fourth optical waveguide1033. The second two-by-two optical splitter 1037 has at least oneoptical output 1037C optically connected to the optical output 1003B ofthe polarization controller 1003. In some embodiments, the at least oneoptical output 1037C of the second two-by-two optical splitter 1037 isthe optical output 1003B of the polarization controller 1003. The secondtwo-by-two optical splitter 1037 is configured to output some of thefirst portion of the incoming light and some of the polarization-rotatedsecond portion of the incoming light through the optical output 1037C.The second two-by-two optical splitter 1037 is not required to be aneven 50-50 optical splitter. The optical splitting ratio of the secondtwo-by-two optical splitter 1037 defines how much optical power istransmitted to the optical output 1037C from each of the first opticalinput 1037A and the second optical input 1037B. The optical splittingratio provided by the second two-by-two optical splitter 1037 is setand/or controlled to ensure that optical power transmission through theoptical output 1037C is optimized for the wavelength channels of theincoming light received through the first optical input 1037A and thesecond optical input 1037B. Also, in some embodiments, the secondtwo-by-two optical splitter 1037 is a non-broadband optical splitter. Insome embodiments, the second two-by-two optical splitter 1037 isimplemented using an MMI device or a directional waveguide coupler,e.g., an adiabatic directional coupler.

In some embodiments, a second phase shifter 1035 is optically coupled toeither the third optical waveguide 1031 or the fourth optical waveguide1033. The example polarization controller 1003 has the second phaseshifter 1035 optically coupled to the second optical waveguide 1033. Insome embodiments, the second phase shifter 1035 is implemented as athermal tuner (e.g., heating device) positioned over the fourth opticalwaveguide 1033, which operates by exploiting the thermo-optic effect ofthe fourth optical waveguide 1033 material. In some embodiments, thesecond phase shifter 1035 is implemented as an electro-optic device(e.g., diode) built into the fourth optical waveguide 1033, whichoperates by exploiting electro-optic effects within the fourth opticalwaveguide 1033. In some embodiments, the second phase shifter 1035 isimplemented as one or more ring resonators, in which each of these ringresonators operates at a particular wavelength to shift the phase oflight at the particular wavelength within the optical waveguide to whichthe second phase shifter 1033 is optically coupled. In some embodiments,the relative phase between the two phase shifters 1027 and 1035 iscontrolled by placing phase shifters on the two optical waveguides 1025and 1033, respectively, instead of on just one of the opticalwaveguides. This provides for faster tuning of the relative phase,especially for thermal phase shifters.

In some embodiments, the polarization controller 1003 functions as aneffective electro-optic combiner by using the PSR 1021 and the cascadedconfiguration of the first two-by-two optical splitter 1029 and thesecond two-by-two optical splitter 1037, with the first phase shifter1027 on one of the two waveguides 1023, 1025 entering the firsttwo-by-two optical splitter 1029, and with the second phase shifter 1035on one of the two waveguides 1031, 1033 entering the second two-by-twooptical splitter 1037. The first phase shifter 1027 and the second phaseshifter 1035 are tuned to account for the phase and intensity imbalanceof the two respective optical waveguides over time. The first phaseshifter 1027 and the second phase shifter 1035 are used to optimize theoptical power in the output waveguide 1011 as the input fiber/waveguide1007 polarization changes over time. In some embodiments, the firstphase shifter 1027 and/or the second phase shifter 1035 are/is implementas a heater placed near the respective optical waveguide or as a diodebuilt into the respective optical waveguide. Also, in some embodiments,the first phase shifter 1027 and/or the second phase shifter 1035 are/isimplemented as a ring resonator phase shifter, in which each of aplurality of ring resonators is operated to shift the phase of a singlerespective wavelength channel of the light within the optical waveguide.Implementation of the first phase shifter 1027 and the second phaseshifter 1035 as ring resonator phase shifters provides for higheroptical power transmission to the output waveguide 1011 over a widerange of wavelength channels. In some situations, as the inputfiber/waveguide 1007 polarization drifts enough over time, the firstphase shifter 1027 and the second phase shifter 1035 may have to “reset”by abruptly changing the phase by a 2π amount to avoid reaching the endof its range. Such a “reset” would take time and cause an interruptionin signal. In some embodiments, to avoid having a “reset” of the phaseshifters 1027, 1035, the polarization controller 1003 includes more thantwo cascaded two-by-two optical splitters with corresponding precedingphase shifters. Also, in some embodiments, the feedback circuitry 1015is configured to control the first phase shifter 1027 and the secondphase shifter 1035, and any other phase shifters in the polarizationcontroller 1003, based on the light transmitted through the opticaloutput waveguide 1011.

The optical input polarization management device 1000 functions toconvert an incoming light signal that has unknown polarizationcharacteristics (and possibly uncontrolled polarization states that varyover time) into a corresponding input light signal of knownpolarization. Because the optical outputs of the PSR 1021 are combinedinto the single output waveguide 1011, with the same polarization, it ispossible to simplify optical circuits and electrical circuits fordetection and/or modulation of the light within the output waveguide1011. In some embodiments, the optical input polarization managementdevice 1000 is implemented as an electro-optic combiner to combine twooptical signals having relative phase and relative intensities that areunknown and that may vary over time, with low loss, over a range ofwavelength channels. In some embodiments, when the optical inputpolarization management device 1000 is used to combine modulated lightsignals for output to a detection system, optical timing-skew managementand/or electrical timing-skew management can be implemented inconjunction with the optical input polarization management device 1000to support receipt of the incoming optical signal.

FIG. 10C shows an example implementation of the optical inputpolarization management device 1000 in which the first phase shifter1027 is implemented as a first plurality of ring resonator phaseshifters 1041-1 to 1041-3 and the second phase shifter 1035 isimplemented as a second plurality of ring resonator phase shifters1043-1 to 1043-3, in accordance with some embodiments. Each of the firstplurality of ring resonator phase shifters 1043-1 to 1043-3 ispositioned along the optical waveguide 1025 and within an evanescentoptical coupling distance of the optical waveguide 1025. Each of thesecond plurality of ring resonator phase shifters 1043-1 to 1043-3 ispositioned along the optical waveguide 1033 and within an evanescentoptical coupling distance of the optical waveguide 1033. Each of thefirst plurality of ring resonator phase shifters 1041-1 to 1041-3 isoperated to provide a controlled amount of shift in the phase of asingle, respective wavelength channel of the light within the opticalwaveguide 1025. Each of the second plurality of ring resonator phaseshifters 1043-1 to 1043-3 is operated to provide a controlled amount ofshift in the phase of a single, respective wavelength channel of thelight within the optical waveguide 1033. A number of ring resonatorswithin the first plurality of ring resonator phase shifters 1041-1 to1041-3 is equal to a number of ring resonators within the secondplurality of ring resonator phase shifters 1043-1 to 1043-3. Also, forfeedback control purposes, the number of ring resonator filters(photodetectors) 1013-1 to 1013-3 is equal to the number of firstplurality of ring resonator phase shifters 1041-1 to 1041-3, and thenumber of ring resonator filters (photodetectors) 1013-1 to 1013-3 isalso equal to the number of the second plurality of ring resonator phaseshifters 1043-1 to 1043-3.

In some embodiments, the ring resonators within the first plurality ofring resonator phase shifters 1041-1 to 1041-3 and the second pluralityof ring resonator phase shifters 1043-1 to 1043-3 are implemented asannular-shaped waveguides having circuitous configuration, e.g.,circular, oval, race-track, or another arbitrary circuitous shape. Insome embodiments, the ring resonators within the first plurality of ringresonator phase shifters 1041-1 to 1041-3 and the second plurality ofring resonator phase shifters 1043-1 to 1043-3 are implemented ascircular discs. The ring resonators within the first plurality of ringresonator phase shifters 1041-1 to 1041-3 and the second plurality ofring resonator phase shifters 1043-1 to 1043-3 are formed of a materialthrough which light can be in-coupled, out-coupled, and guided. Each ofthe ring resonators within the first plurality of ring resonator phaseshifters 1041-1 to 1041-3 and the second plurality of ring resonatorphase shifters 1043-1 to 1043-3 is formed within a surrounding materialthat has an optical index of refraction sufficiently different from thatof the ring resonators to enable guiding of light within the ringresonators and around the circuitous path defined by each of the ringresonators. In some embodiments, each of the first plurality of ringresonator phase shifters 1041-1 to 1041-3 and the second plurality ofring resonator phase shifters 1043-1 to 1043-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the first plurality of ringresonator phase shifters 1041-1 to 1041-3 and the second plurality ofring resonator phase shifters 1043-1 to 1043-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 10micrometers.

While the example of FIG. 10C shows three ring resonators within each ofthe first plurality of ring resonator phase shifters 1041-1 to 1041-3and the second plurality of ring resonator phase shifters 1043-1 to1043-3 for purposes of description, it should be understood that thereis no limit on the number of ring resonators within each of the firstplurality of ring resonator phase shifters 1041-1 to 1041-3 and thesecond plurality of ring resonator phase shifters 1043-1 to 1043-3, solong as the first plurality of ring resonator phase shifters 1041-1 to1041-3 and the second plurality of ring resonator phase shifters 1043-1to 1043-3 and associated signal processing circuitry can be spatiallyand electrically accommodated on the chip. Each ring resonator withinthe first plurality of ring resonator phase shifters 1041-1 to 1041-3and the second plurality of ring resonator phase shifters 1043-1 to1043-3 is configured to operate at a respective resonant wavelength λ₁to λ₃, such that light having a wavelength (λ₁, λ₂, or λ₃) substantiallyequal to the respective resonant wavelength (λ₁, λ₂, or λ₃) of a givenone of the ring resonators optically couples into the given one of thering resonators from the optical waveguide 1025, 1033. Each of the firstplurality of ring resonator phase shifters 1041-1 to 1041-3, the secondplurality of ring resonator phase shifters 1043-1 to 1043-3, and thering resonator filters 1013-1 to 1013-3 includes at least one ringresonator configured to operate at a same resonant wavelength (λ₁, λ₂,or λ₃) corresponding to a channel wavelength within the incoming lightsignal. Also, each of the first plurality of ring resonator phaseshifters 1041-1 to 1041-3, the second plurality of ring resonator phaseshifters 1043-1 to 1043-3, and the ring resonator filters 1013-1 to1013-3 includes at least one ring resonator configured to operate ateach of multiple different resonant wavelengths (λ₁, λ₂, or λ₃)corresponding to the channel wavelengths within the incoming lightsignal. It should be appreciated that first plurality of ring resonators1041-1 to 1041-3 and the second plurality of ring resonators 1043-1 to1043-3 enables different wavelength channels to be phase shiftedseparately to accommodate unique phase and intensity imbalances amongthe different wavelength channels of the incoming light signal.

FIG. 11 shows a flowchart of a method for optical input polarizationmanagement, in accordance with some embodiments. In some embodiments,the method of FIG. 11 is practiced using the optical input polarizationmanagement device 1000 of FIGS. 10A to 10C. The method includes anoperation 1101 for receiving incoming light through an optical inputport (e.g., 1003A) of a PIC (e.g., 1001). A first portion of theincoming light has a first polarization and a second portion of theincoming light has a second polarization. The method also includes anoperation 1103 for splitting the first portion of the incoming lightfrom the second portion of the incoming light. The method also includesan operation 1105 for directing the first portion of the incoming lightthrough a first optical waveguide (e.g., 1023) and into a first opticalinput (e.g., 1029A) of a first two-by-two splitter (e.g., 1029). Themethod also includes an operation 1107 for rotating the secondpolarization of the second portion of the incoming light to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes an operation 1109 for directing the polarization-rotatedsecond portion of the incoming light through a second optical waveguide(e.g., 1025) and into a second optical input (e.g., 1029B) of the firsttwo-by-two splitter. The method also includes an operation 1111 foroperating a first phase shifter (e.g., 1027) in optical coupling witheither the first optical waveguide or the second optical waveguide toapply a controlled amount of shift to a phase of light traveling througheither the first optical waveguide or the second optical waveguide towhich the phase shifter is optically coupled.

The method also includes an operation 1113 for directing some of thefirst portion of the incoming light through a first optical output(e.g., 1029C) of the first two-by-two optical splitter and into a thirdoptical waveguide (e.g., 1031). The method also includes an operation1115 for directing some of the first portion of the incoming lightthrough a second optical output (e.g., 1029D) of the first two-by-twooptical splitter and into a fourth optical waveguide (e.g., 1033). Themethod also includes an operation 1117 for directing some of thepolarization-rotated second portion of the incoming light through thefirst optical output of the first two-by-two optical splitter and intothe third optical waveguide. The method also includes an operation 1119for directing some of the polarization-rotated second portion of theincoming light through the second optical output of the first two-by-twooptical splitter and into the fourth optical waveguide. The method alsoincludes an operation 1121 for operating a second phase shifter (e.g.,1035) in optical coupling with either the third optical waveguide or thefourth optical waveguide to apply a controlled amount of shift to aphase of light traveling through either the third optical waveguide orthe fourth optical waveguide to which the phase shifter is opticallycoupled.

The method also includes an operation 1123 for directing some of thefirst portion of the incoming light and some of the polarization-rotatedsecond portion of the incoming light from the third optical waveguideinto a first optical input (e.g., 1037A) of a second two-by-two splitter(e.g., 1037). The method also includes an operation 1125 for directingsome of the first portion of the incoming light and some of thepolarization-rotated second portion of the incoming light from thefourth optical waveguide into a second optical input (e.g., 1037B) ofthe second two-by-two splitter. The method also includes an operation1127 for directing some of the first portion of the incoming light andsome of the polarization-rotated second portion of the incoming lightthrough an optical output (e.g., 1037C) of the second two-by-twosplitter and into a fifth optical waveguide (e.g., into the outputoptical waveguide 1011).

In some embodiments, the method of FIG. 11 includes operating aplurality of ring resonator photodetectors (e.g., 1013-1 to 1013-3) toevanescently in-couple light from the fifth optical waveguide, whereeach of the plurality of ring resonator photodetectors is operated at arespective resonant wavelength to in-couple a fraction of the firstportion of the incoming light having the respective resonant wavelengthand a fraction of the polarization-rotated second portion of theincoming light having the respective resonant wavelength. In someembodiments of the method of FIG. 11 , the first phase shifter includesa first plurality of ring resonator phase shifters (e.g., 1041-1 to1041-3), and the second phase shifter includes a second plurality ofring resonator phase shifters (e.g., 1043-1 to 1043-3). In theseembodiments, the ring resonator photodetectors are used to generatefeedback signals to control respective ones of the ring resonator phaseshifters in the first phase shifter and the second phase shifter.

FIG. 12 shows an example configuration of an electro-optic transmitter1200 implemented within a PIC 1201, in accordance with some embodiments.The electro-optic transmitter 1200 implements multiple (N) instances ofthe polarization controller 1003 as previously described with regard toFIGS. 10A-10C and 11 . Each instance of the polarization controller1003-x, where x is 1 to N, has the optical input 1003A opticallyconnected to receive incoming light from a respective optical coupler1203-x, where x is 1 to N, by way of an optical waveguide 1204-x, wherex is 1 to N. In some embodiments, the optical input 1003A of eachinstance of the polarization controller 1003-x is directly opticallycoupled to the respective optical coupler 1203-x, such that the opticalwaveguide 1204-x is not required. In some embodiments, the opticalcoupler 1203-x is implemented as an edge coupler. However, in otherembodiments, the optical coupler 1203-x is implemented as a verticalgrating coupler, or as another type of optical coupling device thatprovides for optical coupling of the polarization controller 1003-x to arespective optical fiber/waveguide 1205-x, where x is 1 to N. Incominglight is transmitted from the optical fiber/waveguide 1205-x into theoptical coupler 1203-x, as indicated by arrow 1206-x, where x is 1 to N.Each of the optical fiber/waveguides 1205-1 to 1205-N is opticallyconnected to receive and convey light from a respectivesingle-wavelength light source 1207-1 to 1207-N. In some embodiments,the different single-wavelength light sources 1207-1 to 1207-N areconfigured to supply respectively different wavelengths of continuouswave laser light. In some embodiments, a polarization of the lighttransmitted by the single-wavelength light sources 1207-1 to 1207-Nthrough the respective optical fibers/waveguides 1205-1 to 1205-N isuncontrolled.

Each of the on-chip polarization controllers 1003-1 to 1003-N isconfigured to combine the two polarizations (TE and TM) of the incominglight as received through the respective optical coupler 1203-1 to1203-N as a single polarization of light and output the singlepolarization of light through a respective optical waveguide 1209-1 to1209-N in a low loss manner. For example, in some embodiments, a giveninstance of the polarization controller 1003-x is configured to receiveboth TE and TM polarizations of light through the optical coupler1203-x, rotate the TE polarized light to TM polarized light, andtransmit essentially all of the light received through the opticalcoupler 1203-x as TM polarized light through the optical waveguide1209-x. Alternatively, in some embodiments, a given instance of thepolarization controller 1003-x is configured to receive both TE and TMpolarizations of light through the optical coupler 1203-x, rotate the TMpolarized light to TE polarized light, and transmit essentially all ofthe light received through the optical coupler 1203-x as TE polarizedlight through the optical waveguide 1209-x. In some embodiments, eachinstance of the polarization controller 1003-x is electronically tunableto accommodate a power difference and a phase difference between the twopolarizations (TE and TM) within the incoming light that are unknown andpossibly varying with time. It should be appreciated that because eachof the polarization controllers 1003-1 to 1003-N can be optimized for asingle wavelength, the configuration of the electro-optic transmitter1200 advantageously overcomes any limitation associated with individualones of the polarization controllers 1003-1 to 1003-N having a finiteoptical bandwidth.

The electro-optic transmitter 1200 also includes an optical multiplexer1211 having a plurality of optical inputs 1211A-1 to 1211A-Nrespectively optically connected to the optical waveguides 1209-1 to1209-N corresponding to the plurality of polarization controllers 1003-1to 1003-N. The optical multiplexer 1211 also has a plurality of opticaloutputs 1211B-1 to 1211B-N. The optical multiplexer 1211 is configuredto convey a portion of the light received through any given one of theoptical inputs 1211A-1 to 1211A-N to each of the optical outputs 1211B-1to 1211B-N. In this manner, a portion of the light received through eachof the optical inputs 1211A-1 to 1211A-N is conveyed to each of theoptical outputs 1211B-1 to 1211B-N. Therefore, with thesingle-wavelength light sources 1207-1 to 1207-N respectively supplyingN different wavelengths (λ₁ to λ_(N)) of light, each of the opticalinputs 1211A-1 to 1211A-N receives a different one of the N differentwavelengths (λ₁ to λ_(N)) of light (having a single, controlledpolarization (either TE or TM)), and the optical multiplexer 1211functions to convey a portion of each of the N different wavelengths (λ₁to λ_(N)) of light from each of the optical inputs 1211A-1 to 1211A-N toeach of the optical outputs 1211B-1 to 1211B-N, such that all of the Ndifferent wavelengths (λ₁ to λ_(N)) of light (having the single,controlled polarization (either TE or TM)) are conveyed through each ofthe optical outputs 1211B-1 to 1211B-N. In some embodiments, the opticalmultiplexer 1211 is implemented as a star coupler. In some embodiments,the optical multiplexer 1211 is implemented as a series of cascadedtwo-by-two optical splitters. It should be understood, however, that inother embodiments, the optical multiplexer 1211 can be implemented inother ways so long as the above-mentioned functionality of the opticalmultiplexer 1211 is achieved.

The electro-optic transmitter 1200 also includes a plurality of opticalwaveguides 1213-1 to 1213-N, where each of the optical waveguides 1213-1to 1213-N has a first end optically connected to a respective one of theplurality of optical outputs 1211B-1 to 1211B-N of the opticalmultiplexer 1211, and where each of the optical waveguides 1213-1 to1213-N has a second end optically connected to a respective opticaloutput port of the electro-optic transmitter 1200. In some embodiments,the optical output ports of the electro-optic transmitter 1200 areimplemented as optical couplers 1219-1 to 1219-N. In some embodiments,each instance of the optical coupler 1219-x, where x is 1 to N, isimplemented as an edge coupler. However, in other embodiments, theoptical coupler 1219-x is implemented as a vertical grating coupler, oras another type of optical coupling device that provides for opticalcoupling of the corresponding optical waveguide 1213-1 to 1213-N to arespective output optical fiber/waveguide connected/couple to theelectro-optic transmitter 1200.

In some embodiments, the electro-optic transmitter 1200 includes aplurality of ring resonator modulators 1215-x-y positioned along andwithin an evanescent optical coupling distance of each of the pluralityof optical waveguides 1213-1 to 1213-N, where x is 1 to N, and y is 1 toY. While the example electro-optic transmitter 1200 shows three ringresonator modulators 1215-x-1 to 1215-x-3 along each of the opticalwaveguides 1213-1 to 1213-N for purposes of description, it should beunderstood that there is no limit on the number of ring resonatormodulators that can be positioned along each of the optical waveguides1213-1 to 1213-N, so long as the ring resonator modulators andassociated signal processing circuitry can be spatially and electricallyaccommodated on the chip. In some embodiments, the ring resonatormodulators 1215-x-y are implemented as annular-shaped waveguides havingcircuitous configuration, e.g., circular, oval, race-track, or anotherarbitrary circuitous shape. In some embodiments, the ring resonatormodulators 1215-x-y are implemented as circular discs. The ringresonator modulators 1215-x-y are formed of a material through whichlight can be in-coupled, out-coupled, and guided. Each of the ringresonator modulators 1215-x-y is formed within a surrounding materialthat has an optical index of refraction sufficiently different from thatof the ring resonator modulators 1215-x-y to enable guiding of lightwithin the ring resonator modulators 1215-x-y and around the circuitouspath defined by each of the ring resonator modulators 1215-x-y. In someembodiments, each of the ring resonator modulators 1215-x-y isconfigured to have an annular-shape or disc-shape with an outer diameterof less than about 50 micrometers. In some embodiments, each of the ringresonator modulators 1215-x-y is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 10 micrometers.

Each of the ring resonator modulators 1215-x-1 to 1215-x-3 is configuredto operate at a respective resonant wavelength λ₁ to λ₃, such that thefirst portion of the incoming light and the polarization-rotated secondportion of the incoming light having a wavelength (λ₁, λ₂, or λ₃)substantially equal to the respective resonant wavelength (λ₁, λ₂, orλ₃) of a given one of the ring resonator modulators 1215-x-1 to 1215-x-3optically couples into the given one of the ring resonator modulators1215-x-1 to 1215-x-3 from the corresponding optical waveguide 1213-x.Each of the ring resonator modulators 1215-x-y operates to modulatelight of a particular wavelength within the corresponding opticalwaveguide 1213-x to convey a digital data. In some embodiments, the ringresonator modulators 1215-x-y include photodetector devices to enablemonitoring of the optical power coupled into each of the ring resonatormodulators 1215-x-y. In some embodiments, the optical power measured byphotodetectors within the ring resonator modulators 1215-x-y is by thefeedback circuitry 1015 to control the polarization controllers 1003-1to 1003-N.

FIG. 13 shows a flowchart of a method for operating an electro-optictransmitter, in accordance with some embodiments. In some embodiments,the method of FIG. 13 is practiced using the electro-optic transmitter1200 of FIG. 12 . The method includes an operation 1301 for receivingincoming light through a plurality of optical input ports (e.g., opticalcouplers 1203-1 to 1203-N). In some embodiments, the incoming lightreceived through any given one of the plurality of optical input portsis continuous wave laser light of a single wavelength (e.g., any one ofwavelengths λ₁ to λ_(N)). Also, the incoming light received throughdifferent ones of the plurality of optical input ports has differentwavelengths. In this manner, each of the different optical input portsreceives a different wavelength of continuous wave laser light. Themethod also includes an operation 1303 for operating a plurality ofpolarization controllers (e.g., polarization controllers 1003-1 to1003-N). Each of the plurality of polarization controllers has anoptical input respectively optically connected to the plurality ofoptical input ports. Each of the plurality of polarization controllersoperates to convert light having two polarizations (TE and TM) asreceived through a corresponding one of the plurality of optical inputports into a light having a single polarization (TE or TM). Each of theplurality of polarization controllers operates to direct the lighthaving the single polarization through an output optical waveguide(e.g., 1209-1 to 1209-N) of the polarization controller. In someembodiments, each of the plurality of polarization controllers isoperated in accordance with the method of FIG. 11 .

The method also includes an operation 1305 for operating an opticalmultiplexer (e.g., 1211) having a plurality of optical inputs (e.g.,1211A-1 to 1211A-N) respectively optically connected to the outputoptical waveguides of the plurality of polarization controllers. Theoptical multiplexer has a plurality of optical outputs (e.g., 1211B-1 to1211B-N). The optical multiplexer operates to direct a portion of lightreceived at each of the plurality of optical inputs of the opticalmultiplexer to each of the plurality of optical outputs of the opticalmultiplexer. The method also includes an operation 1307 for directinglight from each of the plurality of optical outputs of the opticalmultiplexer through respective ones of a plurality of optical waveguides(e.g., optical waveguides 1213-1 to 1213-N). Each of the plurality ofoptical waveguides has a first end and second end. The first end of eachof the plurality of optical waveguides is respectively opticallyconnected to the plurality of optical outputs of the opticalmultiplexer. The second end of each of the plurality of opticalwaveguides is respectively optically connected to a plurality of opticaloutput ports (e.g., 1219-1 to 1219-N). The method also includes anoperation 1309 for operating a plurality of ring resonator modulators(e.g., 1215-x-y) positioned along a given one of the plurality ofoptical waveguides to modulate light within the given one of theplurality of optical waveguides in accordance with digital data. In someembodiments, a separate plurality of ring resonator modulators ispositioned along each of the plurality of optical waveguides, where eachof the separate pluralities of ring resonator modulators are operated tomodulate light within the corresponding optical waveguide in accordancewith digital data.

FIG. 14 shows an example configuration of an electro-optic transmitter1400 implemented within a PIC 1401, in accordance with some embodiments.The electro-optic transmitter 1400 includes a first PSR 1403 that has anoptical input 1403A optically connected to receive incoming light froman optical coupler 1405 through an optical waveguide 1406. In someembodiments, the optical input 1403A of the PSR 1403 is directlyoptically coupled to the optical coupler 1405, such that the opticalwaveguide 1406 is not required. In some embodiments, the optical coupler1405 is implemented as an edge coupler. However, in other embodiments,the optical coupler 1405 is implemented as a vertical grating coupler,or as another type of optical coupling device that provides for opticalcoupling of the PIC 1401 to an optical fiber/waveguide 1407. Incominglight is transmitted from the optical fiber/waveguide 1407 into theoptical coupler 1405, as indicated by arrow 1408. The opticalfiber/waveguide 1407 is optically connected to receive and convey lightfrom a multi-wavelength light source 1409. In some embodiments, themulti-wavelength light source 1409 is configured to transmit multiplewavelengths of continuous wave laser light through the opticalfiber/waveguide 1407. In some embodiments, a polarization of the lighttransmitted by the multi-wavelength light source 1409 through theoptical fiber/waveguide 1407 is uncontrolled and possibly varies overtime.

The PSR 1403 has a first optical output 1403B and a second opticaloutput 1403C. The PSR 1403 is configured to direct a first portion ofthe incoming light having a first polarization (TE or TM) through thefirst optical output 1403B. The PSR 1403 is also configured to rotate apolarization of a second portion of the incoming light from a secondpolarization (opposite of the first polarization) to the firstpolarization. In this manner, the PSR 1403 turns the second portion ofthe incoming light into a polarization-rotated second portion of theincoming light. The PSR 1403 is configured to direct thepolarization-rotated second portion of the incoming light through thesecond optical output 1403C.

The electro-optic transmitter 1400 includes a first optical waveguide1411 optically connected to the first optical output 1403B of the PSR1403. The electro-optic transmitter 1400 also includes a second opticalwaveguide 1413 optically connected to the second optical output 1403C ofthe PSR 1403. The first optical waveguide 1411 and the second opticalwaveguide 1413 are formed of a material through which light can bein-coupled, out-coupled, and guided. Each of the first optical waveguide1411 and the second optical waveguide 1413 is formed within asurrounding material that has an optical index of refractionsufficiently different from that of the first optical waveguide 1411 andthe second optical waveguide 1413, respectively, to enable guiding oflight within the first optical waveguide 1411 and the second opticalwaveguide 1413. In some embodiments, first optical waveguide 1411 andthe second optical waveguide 1413 are formed of a same material. In someembodiments, the first portion of the incoming light is transmittedthrough the first optical output 1403B of the PSR 1403 and into thefirst optical waveguide 1411, and travels along the first opticalwaveguide 1411, as indicated by arrows 1412. Also, in these embodiments,the polarization-rotated second portion of the incoming light istransmitted through the second optical output 1403C of the PSR 1403 andinto the second optical waveguide 1413, and travels along the secondoptical waveguide 1413, as indicated by arrows 1414. Alternatively, insome embodiments, the first portion of the incoming light is transmittedthrough the second optical output 1403C of the PSR 1403 and into thesecond optical waveguide 1413, and travels along the second opticalwaveguide 1413, as indicated by arrows 1414. Also, in these alternativeembodiments, the polarization-rotated second portion of the incominglight is transmitted through the first optical output 1403B of the PSR1403 and into the first optical waveguide 1411, and travels along thefirst optical waveguide 1411, as indicated by arrows 1412.

The electro-optic transmitter 1400 includes a first plurality (array) ofring resonator modulators 1415-1 to 1415-3 positioned along the firstoptical waveguide 1411 and within an evanescent optical couplingdistance of the first optical waveguide 1411. While the exampleelectro-optic transmitter 1400 shows three ring resonator modulators1415-1 to 1415-3 for purposes of description, it should be understoodthat there is no limit on the number of ring resonator modulators in thefirst plurality of ring resonator modulators 1415-1 to 1415-3 that canbe positioned along the first optical waveguide 1411, so long as thefirst plurality of ring resonator modulators 1415-1 to 1415-3 andassociated signal processing circuitry can be spatially and electricallyaccommodated on the chip. Each of the ring resonator modulators 1415-1to 1415-3 is configured to operate at a respective resonant wavelengthλ₁ to λ₃, such that the first portion of the incoming light having awavelength (λ₁, λ₂, or λ₃) substantially equal to the respectiveresonant wavelength (λ₁, λ₂, or λ₃) of a given one of the ring resonatormodulators 1415-1 to 1415-3 optically couples into the given one of thering resonator modulators 1415-1 to 1415-3 from the first opticalwaveguide 1411. In some embodiments, the ring resonator modulators1415-1 to 1415-3 are implemented as annular-shaped waveguides havingcircuitous configuration, e.g., circular, oval, race-track, or anotherarbitrary circuitous shape. In some embodiments, the ring resonatormodulators 1415-1 to 1415-3 are implemented as circular discs. The ringresonator modulators 1415-1 to 1415-3 are formed of a material throughwhich light can be in-coupled, out-coupled, and guided. Each of the ringresonator modulators 1415-1 to 1415-3 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the ring resonator modulators 1415-1 to 1415-3 to enableguiding of light within the ring resonator modulators 1415-1 to 1415-3and around the circuitous path defined by each of the ring resonatorsresonator modulators 1415-1 to 1415-3. In some embodiments, each of thering resonator modulators 1415-1 to 1415-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonator modulators1415-1 to 1415-3 is configured to have an annular-shape or disc-shapewith an outer diameter of less than about 10 micrometers.

The electro-optic transmitter 1400 also includes a second plurality(array) of ring resonator modulators 1417-1 to 1417-3 positioned alongthe second optical waveguide 1413 and within an evanescent opticalcoupling distance of the second optical waveguide 1413. While theexample electro-optic transmitter 1400 shows three ring resonatormodulators 1417-1 to 1417-3 for purposes of description, it should beunderstood that there is no limit on the number of ring resonatormodulators in the second plurality of ring resonator modulators 1417-1to 1417-3 that can be positioned along the second optical waveguide1413, so long as the second plurality of ring resonator modulators1417-1 to 1417-3 and associated signal processing circuitry can bespatially and electrically accommodated on the chip. Each of the ringresonator modulators 1417-1 to 1417-3 is configured to operate at arespective resonant wavelength λ₁ to λ₃, such that thepolarization-rotated second portion of the incoming light having awavelength (λ₁, λ₂, or λ₃) substantially equal to the respectiveresonant wavelength (λ₁, λ₂, or λ₃) of a given one of the ring resonatormodulators 1417-1 to 1417-3 optically couples into the given one of thering resonator modulators 1417-1 to 1417-3 from the second opticalwaveguide 1413. In some embodiments, the ring resonator modulators1417-1 to 1417-3 are implemented as annular-shaped waveguides havingcircuitous configuration, e.g., circular, oval, race-track, or anotherarbitrary circuitous shape. In some embodiments, the ring resonatormodulators 1417-1 to 1417-3 are implemented as circular discs. The ringresonator modulators 1417-1 to 1417-3 are formed of a material throughwhich light can be in-coupled, out-coupled, and guided. Each of the ringresonator modulators 1417-1 to 1417-3 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the ring resonator modulators 1417-1 to 1417-3 to enableguiding of light within the ring resonator modulators 1417-1 to 1417-3and around the circuitous path defined by each of the ring resonatorsresonator modulators 1417-1 to 1417-3. In some embodiments, each of thering resonator modulators 1417-1 to 1417-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonator modulators1417-1 to 1417-3 is configured to have an annular-shape or disc-shapewith an outer diameter of less than about 10 micrometers.

The first plurality of ring resonator modulators 1415-1 to 1415-3 andthe second plurality of ring resonator modulators 1417-1 to 1417-3 forma plurality of ring resonator modulator pairs positioned along the firstoptical waveguide 1411 and the second optical waveguide 1413. Each ringresonator modulator pair of the plurality of ring resonator modulatorpairs includes one ring resonator modulator (one of 1415-1 to 1415-3)positioned within an evanescent optical coupling distance of the firstoptical waveguide 1411 and one ring resonator modulator (one of 1417-1to 1417-3) positioned within an evanescent optical coupling distance ofthe second optical waveguide 1413, where each of the plurality of ringresonator modulator pairs is configured to operate at a specifiedresonant wavelength (one of λ₁ to λ₃). Each ring resonator modulatorwithin a given one of the plurality of ring resonator modulator pairs isconfigured to modulate a same bit pattern. For example, the pair of ringresonator modulators 1415-1 and 1417-1 is configured to modulate a samebit pattern. The pair of ring resonator modulators 1415-2 and 1417-2 isconfigured to modulate a same bit pattern. And, the pair of ringresonator modulators 1415-3 and 1417-3 is configured to modulate a samebit pattern, and so on.

The electro-optic transmitter 1400 includes a second PSR 1419 that has areverse-connected optical input 1419A, a first reverse-connected opticaloutput 1419B, and a second reverse-connected optical output 1419C. Thesecond PSR 1419 is connected in a reversed manner in the electro-optictransmitter 1400, such that the first reverse-connected optical output1419B and the second reverse-connected optical output 1419C areconnected to function as respective optical inputs, and such that thereverse-connected optical input 1419A is connected to function as anoptical output. In this manner, the second PSR 1419 functions as apolarization rotator and optical combiner. Specifically, the firstreverse-connected optical output 1419B of the second PSR 1419 isoptically connected to a second end of the first optical waveguide 1411,such that light conveyed through the first optical waveguide 1411 isreceived as input light into the first reverse-connected optical output1419B. Also, the second reverse-connected optical output 1419C of thesecond PSR 1419 is optically connected to a second end of the secondoptical waveguide 1413, such that light conveyed through the secondoptical waveguide 1413 is received as input light into the secondreverse-connected optical output 1419C. The reverse-connected opticalinput 1419A of the PSR 1419 is optically connected to output coupler1421 of the electro-optic transmitter 1400 through an optical waveguide1423. In this manner, the reverse-connected optical input 1419A actuallyoperates as an optical output through which light is transmitted fromthe PSR 1419 through the optical waveguide 1423 to the output coupler1421. Modulated output light is transmitted through the optical coupler1421, as indicated by arrow 1425. In some embodiments, thereverse-connected optical input 1419A of the PSR 1419 is directlyoptically coupled to the optical coupler 1421, such that the opticalwaveguide 1423 is not required. In some embodiments, the optical coupler1421 is implemented as an edge coupler. However, in other embodiments,the optical coupler 1421 is implemented as a vertical grating coupler,or as another type of optical coupling device that provides for opticalcoupling of the PIC 1401 to an optical fiber/waveguide.

In a reverse functional manner, the PSR 1419 is configured to directmodulated light based on the first portion of the incoming light (havingthe first polarization) as received from the first optical waveguide1411 through the first reverse-connected optical output 1419B to thereverse-connected optical input 1419A and on to the optical coupler1421. Also, in a reverse functional manner, the PSR 1419 is configuredto rotate a polarization of modulated light based on thepolarization-rotated second portion of the incoming light (having thefirst polarization), as received from the second optical waveguide 1413through the second reverse-connected optical output 1419C, from thefirst polarization back to the second polarization (opposite of thefirst polarization). In this manner, the PSR 1419 turns the modulatedlight based on the polarization-rotated second portion of the incominglight (having the first polarization) into a polarization-derotatedmodulated light (having the second polarization). The PSR 1419 isconfigured to direct the polarization-derotated modulated light throughthe reverse-connected optical input 1419A and on to the optical coupler1421. The reverse-implemented PSR 1419 allows optical signals to becombined without active phase control, as each optical signal is in adifferent polarization state when it gets combined and transmittedthrough the reverse-connected optical input 1419A of the PSR 1419 as acombined optical output signal which is then output through the opticalcoupler 1421 to an optical fiber/waveguide.

FIG. 15 shows a flowchart of a method for optical modulation, inaccordance with some embodiments. In some embodiments, the method ofFIG. 15 is practiced using the electro-optic transmitter 1400 of FIG. 14. The method includes an operation 1501 for receiving incoming lightthrough an optical input port, where a first portion of the incominglight has a first polarization and a second portion of the incominglight has a second polarization. In some embodiments, the method of FIG.15 is practiced using the electro-optic transmitter 1400 of FIG. 14 . Insome embodiments, the incoming light has an unknown polarization. Insome embodiments, a polarization of the incoming light is uncontrolledand can possibly vary over time. The method includes an operation 1503for splitting the first portion of the incoming light from the secondportion of the incoming light. The method includes an operation 1505 fordirecting the first portion of the incoming light through a firstoptical waveguide (e.g., optical waveguide 1411). The method includes anoperation 1507 for rotating the second polarization of the secondportion of the incoming light to the first polarization so that thesecond portion of the incoming light is a polarization-rotated secondportion of the incoming light. The method includes an operation 1509 fordirecting the polarization-rotated second portion of the incoming lightthrough a second optical waveguide (e.g., optical waveguide 1413). Insome embodiments, the operations 1501 through 1509 are performed by thePSR 1403.

The method includes an operation 1511 for operating a plurality of ringresonator modulator pairs (e.g., pairs of ring resonator modulators1415-1 to 1415-3 and 1417-1 to 1417-3) positioned along the firstoptical waveguide and the second optical waveguide. Each ring resonatormodulator pair of the plurality of ring resonator modulator pairsincludes one ring resonator modulator positioned within an evanescentoptical coupling distance of the first optical waveguide and one ringresonator modulator positioned within an evanescent optical couplingdistance of the second optical waveguide. Each of the plurality of ringresonator modulator pairs is configured to operate at a specifiedresonant wavelength to modulate a same bit pattern onto light travelingthrough the first optical waveguide and the second optical waveguide tocreate a first portion of modulated light having the first polarizationwithin the first optical waveguide and to create a second portion ofmodulated light having the first polarization within the second opticalwaveguide. The method includes an operation 1513 for rotating apolarization of the second portion of modulated light within the secondoptical waveguide back to the second polarization from the firstpolarization. The method also includes an operation 1515 for directingboth the first portion of modulated light having the first polarizationand the second portion of modulated light having the second polarizationthrough a same optical output port (e.g., optical coupler 1421). In someembodiments, the operations 1513 and 1515 are performed by thereverse-implemented PSR 1419.

FIG. 16 shows an example configuration of an electro-optic transmitter1600 implemented within a PIC 1601, in accordance with some embodiments.The electro-optic transmitter 1600 is a variation of the electro-optictransmitter 1400 of FIG. 14 . Specifically, in the electro-optictransmitter 1600, the PSR 1403 of the electro-optic transmitter 1400 isreplaced by the polarization equalizer 812 as previously described withregard to electro-optic receiver 800 of FIG. 8 . The optical input 821Aof the PSR 821 is optically connected to receive the incoming light fromthe optical coupler 1405, either through the optical waveguide 1406 orthrough direct optical coupling of the optical input 821A with theoptical coupler 1405. The first optical output 809C of the two-by-twooptical splitter 809 is optically connected to the first end of thefirst optical waveguide 1411. The second optical output 809D of thetwo-by-two optical splitter 809 is optically connected to the first endof the second optical waveguide 1413.

The electro-optic transmitter 1600 addresses a possible problematicsituation in which either the first optical waveguide 803 or the secondoptical waveguide 805 conveys very little light due to most or all ofthe incoming light, as received through the optical coupler 1405, havingone polarization (either mostly TE or mostly TM). In this situation, ifthe two-by-two optical splitter 809 were not implemented, it would bevery difficult for any ring tuning algorithm to keep the operatingresonant wavelengths of the ring resonator modulators 1415-1 to 1415-3and 1417-1 to 1417-3 aligned with the corresponding channel wavelengths,respectively, in the incoming light signal, as received through theoptical coupler 1405. Also, the above-mentioned situation is even moreproblematic when the polarization in the optical fiber/waveguide 1407evolves over time, because the ring resonator modulators 1415-1 to1415-3 and 1417-1 to 1417-3 will have to re-lock to the channelwavelengths as the optical power ramps up. If the ring resonatormodulators 1415-1 to 1415-3 and 1417-1 to 1417-3 have to re-lock tochanging channel wavelengths, an interruption will occur in the signaloutput by the electro-optic transmitter 1600. To address theabove-mentioned situation, the electro-optic transmitter 1600 implementsthe polarization equalizer 812 that includes the two-by-two opticalsplitter 809 and the phase shifter 807 to ensure non-negligible opticalpower in each of the first optical waveguide 1411 and the second opticalwaveguide 1413 before the light reaches the ring resonator modulators1415-1 to 1415-3 and 1417-1 to 1417-3. The two-by-two optical splitter809 ensures that each of the first optical waveguide 1411 and the secondoptical waveguide 1413 conveys enough light of the first polarization toensure that the ring resonators 1415-1 to 1415-3 and 1417-1 to 1417-3can lock onto and maintain respective resonant wavelengths thatsubstantially align with the channel wavelengths in the incoming lightsignal. In some embodiments, the phase shifter 807 uses active controlas the polarization in the optical fiber/waveguide 1407 drifts overtime. The active control of the phase shifter 807 is implemented byactive control circuitry (feedback circuitry 1015). For example, in someembodiments, active control of the phase shifter 807 is implemented byactive control circuitry that measures optical power in the ringresonator modulators 1415-1 to 1415-3 and 1417-1 to 1417-3, and usesthat measured optical power as feedback signals to adjust the operationof the phase shifter 807 as needed to track with the polarization in theoptical fiber/waveguide 1407.

FIG. 17 shows a flowchart of a method for optical modulation, inaccordance with some embodiments. In some embodiments, the method ofFIG. 17 is practiced using the electro-optic transmitter 1600 of FIG. 16. The method includes an operation 1701 for receiving incoming lightthrough an optical input port (e.g., optical coupler 1405), where afirst portion of the incoming light has a first polarization and asecond portion of the incoming light has a second polarization. Themethod also includes an operation 1703 for splitting the first portionof the incoming light from the second portion of the incoming light. Themethod also includes an operation 1705 for directing the first portionof the incoming light through a first optical waveguide (e.g., opticalwaveguide 1411) and into a first optical input (e.g., 809A) of atwo-by-two splitter (e.g., 809). The method also includes an operation1707 for rotating the second polarization of the second portion of theincoming light to the first polarization so that the second portion ofthe incoming light is a polarization-rotated second portion of theincoming light. The method also includes an operation 1709 for directingthe polarization-rotated second portion of the incoming light through asecond optical waveguide (e.g., optical waveguide 805) and into a secondoptical input (e.g., 809B) of the two-by-two splitter. In someembodiments, the operations 1701 through 1709 are performed by the PSR821.

The method also includes an operation 1711 for directing some of thefirst portion of the incoming light through a first optical output(e.g., 809C) of the two-by-two optical splitter and into a third opticalwaveguide (e.g., the optical waveguide 1411). The method also includesan operation 1713 for directing some of the first portion of theincoming light through a second optical output (e.g., 809D) of thetwo-by-two optical splitter and into a fourth optical waveguide (e.g.,the optical waveguide 1413). The method also includes an operation 1715for directing some of the polarization-rotated second portion of theincoming light through the first optical output (e.g., 809C) of thetwo-by-two optical splitter and into the third optical waveguide (e.g.,1411). The method also includes an operation 1717 for directing some ofthe polarization-rotated second portion of the incoming light throughthe second optical output (e.g., 809D) of the two-by-two opticalsplitter and into the fourth optical waveguide (e.g., 1413).

The method also includes an operation 1719 for operating a plurality ofring resonator modulator pairs (e.g., ring resonator modulators 1415-1to 1415-3 and 1417-1 to 1417-3) positioned along the third opticalwaveguide (e.g., 1411) and the fourth optical waveguide (e.g., 1413),where each ring resonator modulator pair of the plurality of ringresonator modulator pairs includes one ring resonator modulatorpositioned within an evanescent optical coupling distance of the thirdoptical waveguide (e.g., 1411) and one ring resonator modulatorpositioned within an evanescent optical coupling distance of the fourthoptical waveguide (e.g., 1413). Each of the plurality of ring resonatormodulator pairs is configured to operate at a specified resonantwavelength to modulate a same bit pattern onto light traveling throughthe third optical waveguide (e.g., 1411) and the fourth opticalwaveguide (e.g., 1413). The method also includes an operation 1721 forrotating a polarization of modulated light in either the third opticalwaveguide (e.g., 1411) or the fourth optical waveguide (e.g., 1413) fromthe first polarization back to the second polarization. The method alsoincludes an operation 1723 for directing both modulated light having thefirst polarization and modulated light having the second polarizationfrom the third and fourth optical waveguides through a same opticaloutput port (e.g., optical coupler 1421). In some embodiments, theoperations 1721 through 1723 are performed by the reverse-implementedPSR 1419.

FIG. 18 shows an example configuration of an electro-optic combiner 1800implemented within a PIC 1801, in accordance with some embodiments. Theelectro-optic combiner 1800 includes a PSR 1803 that has an opticalinput 1803A optically connected to receive incoming light from anoptical coupler 1805, by way of an optical waveguide 1806. In someembodiments, the optical input 1803A of the PSR 1803 is directlyoptically coupled to the optical coupler 1805, such that the opticalwaveguide 1806 is not required. In some embodiments, the optical coupler1805 is implemented as an edge coupler. However, in other embodiments,the optical coupler 1805 is implemented as a vertical grating coupler,or as another type of optical coupling device that provides for opticalcoupling of the PIC 1801 to an optical fiber/waveguide 1807. Incominglight is transmitted from the optical fiber/waveguide 1807 into theoptical coupler 1805, as indicated by arrow 1808. The PSR 1803 has afirst optical output 1803B and a second optical output 1803C. The PSR1803 is configured to direct a first portion of the incoming lighthaving a first polarization (TE or TM) through the first optical output1803B. The PSR 1803 is also configured to rotate a polarization of asecond portion of the incoming light from a second polarization(opposite of the first polarization) to the first polarization. In thismanner, the PSR 1803 turns the second portion of the incoming light intoa polarization-rotated second portion of the incoming light. The PSR1803 is configured to direct the polarization-rotated second portion ofthe incoming light through the second optical output 1803C.Alternatively, in some embodiments, the first portion of the incominglight having a first polarization is transmitted through the secondoptical output 1803C, and the polarization-rotated second portion of theincoming light is transmitted through the first optical output 1803B.

The electro-optic combiner 1800 includes a first optical waveguide 1809optically connected to the first optical output 1803B of the PSR 1803.The electro-optic combiner 1800 also includes a second optical waveguide1811 optically connected to the second optical output 1803C of the PSR1803. The first optical waveguide 1809 and the second optical waveguide1811 are formed of a material through which light can be in-coupled,out-coupled, and guided. Each of the first optical waveguide 1809 andthe second optical waveguide 1811 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the first optical waveguide 1809 and the second opticalwaveguide 1811, respectively, to enable guiding of light within thefirst optical waveguide 1809 and the second optical waveguide 1811. Insome embodiments, first optical waveguide 1809 and the second opticalwaveguide 1811 are formed of a same material. In some embodiments, thefirst portion of the incoming light is transmitted through the firstoptical output 1803B of the PSR 1803 and into the first opticalwaveguide 1809, and travels along the first optical waveguide 1809, asindicated by arrow 1810. Also, the first polarization-rotated secondportion of the incoming light is transmitted through the second opticaloutput 1803C of the PSR 1803 and into the second optical waveguide 1811,and travels along the second optical waveguide 1811, as indicated byarrow 1812. The first optical waveguide 1809 is configured to reverseits direction into a combiner section 1809A of the first opticalwaveguide 1809. In this manner, light travels through the combinersection 1809A of the first optical waveguide 1809 in a direction, asindicated by arrow 1810A, that is opposite of the direction (arrow 1812)that light travels through the second optical waveguide 1811.Alternatively, in some embodiments, the first portion of the incominglight is transmitted through the second optical output 1803C of the PSR1803 and into the second optical waveguide 1811, and travels along thesecond optical waveguide 1811, as indicated by arrow 1812. Also, inthese alternative embodiments, the polarization-rotated second portionof the incoming light is transmitted through the first optical output1803B of the PSR 1803 and into the first optical waveguide 1809, andtravels along the first optical waveguide 1809 as indicated by arrow1810, and back through the combiner section 1809A of the first opticalwaveguide 1809 as indicated by arrow 1810A.

The electro-optic combiner 1800 also includes a plurality of ringresonators 1813-1 to 1813-3 disposed between the combiner section 1809Aof the first optical waveguide 1809 and a combiner section 1811A of thesecond optical waveguide 1811. While the example electro-optic combiner1800 shows three ring resonators 815-1 to 815-3 for purposes ofdescription, it should be understood that there is no limit on thenumber of these ring resonators, so long as the ring resonators andassociated signal processing circuitry can be spatially and electricallyaccommodated on the chip. In some embodiments, the ring resonators1813-1 to 1813-3 are implemented as annular-shaped waveguides havingcircuitous configuration, e.g., circular, oval, race-track, or anotherarbitrary circuitous shape. In some embodiments, the ring resonators1813-1 to 1813-3 are implemented as circular discs. The ring resonators1813-1 to 1813-3 are formed of a material through which light can bein-coupled, out-coupled, and guided. Each of the ring resonators 1813-1to 1813-3 is formed within a surrounding material that has an opticalindex of refraction sufficiently different from that of the ringresonators 1813-1 to 1813-3 to enable guiding of light within the ringresonators 1813-1 to 1813-3 and around the circuitous path defined byeach of the ring resonators 1813-1 to 1813-3. In some embodiments, eachof the ring resonators 1813-1 to 1813-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonators 1813-1 to1813-3 is configured to have an annular-shape or disc-shape with anouter diameter of less than about 10 micrometers.

Each of the plurality of ring resonators 1813-1 to 1813-3 is positionedwithin an evanescent optically coupling distance of both the combinersection 1809A of the first optical waveguide 1809 and the combinersection 1811A of the second optical waveguide 1811. A light propagationdirection through the combiner section 1809A of the first opticalwaveguide 1809 is opposite of a light propagation direction through thecombiner section 1811A of the second optical waveguide 1811. Each of theplurality of ring resonators 1813-1 to 1813-3 is configured to operateat a respective resonant wavelength (λ₁ to λ₃), such that light having awavelength substantially equal to the respective resonant wavelength ofa given one of the plurality of ring resonators 1813-1 to 1813-3optically couples from the combiner section 1811A of the opticalwaveguide 1811 into the given one of the plurality of ring resonators1813-1 to 1813-3. The light that is coupled into the ring resonators1813-1 to 1813-3 travels around the ring resonators 1813-1 to 1813-3 ina clockwise direction and couples into the combiner section 1811A of thesecond optical waveguide 1811, as indicated by arrows 1814. In thismanner, the light output from the PSR 1803 into the first opticalwaveguide 1809 is combined with the light output by the PSR 1803 intothe second optical waveguide 1811. The combined light within the secondoptical waveguide 1811 after the combiner section 1811A of the secondoptical waveguide 1811 (with respect to the light propagationsdirections 1812 and 1814) is output from the electro-optic combiner1800. In some embodiments, the combined light is output from theelectro-optic combiner 1800 to photodetectors. However, in otherembodiments, the combined light is output from the electro-opticcombiner 1800 to essentially any type of photonic device, as needed. Thering resonators 1813-1 to 1813-3 function as passive filters to combinethe light signals output by the PSR 1803.

Also, the electro-optic combiner 1800 includes a plurality of phaseshifters 1815-1 to 1815-3 optically coupled to the first opticalwaveguide 1809. Each of the plurality of phase shifters 1815-1 to 1815-3is positioned before a respective one of the plurality of ringresonators 1813-1 to 1813-3 with respect to the light propagationdirection 1810A through the combiner section 1809A of the second opticalwaveguide 1809. In this manner, in some embodiments, a number of theplurality of phase shifters 1815-1 to 1815-3 is equal to a number of theplurality of ring resonators 1813-1 to 1813-3. In some embodiments, eachof the phase shifters 1815-1 to 1815-3 is implemented as a thermal tuner(e.g., heating device) positioned over the combiner section 1809A of thefirst optical waveguide 1809, which operates by exploiting thethermo-optic effect of the first optical waveguide 1809 material. Insome embodiments, each of the phase shifters 1815-1 to 1815-3 isimplemented as an electro-optic device (e.g., diode) built into thecombiner section 1809A of the first optical waveguide 1809, whichoperates by exploiting electro-optic effects within the first opticalwaveguide 1809. In some embodiments, each of the phase shifters 1815-1to 1815-3 is implemented as a set of ring resonators. Each of theplurality of phase shifters 1815-1 to 1815-3 is configured to apply acontrolled amount of shift to a phase of light traveling through thefirst optical waveguide 1809. The phase shifters 1815-1 to 1815-3 arecontrolled/operated to ensure proper phase matching between the lightsignals within the first optical waveguide 1809 and the second opticalwaveguide 1811. The phase shifters 1815-1 to 1815-3 are tuned along withthe resonance wavelengths of the ring resonators 1813-1 to 1813-3 toaccount for phase and intensity imbalances over time within the firstoptical waveguide 1809 and the second optical waveguide 1811. In someembodiments, the electro-optic combiner 1800 does not require atiming-skew management system.

FIG. 19 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments. In some embodiments, themethod of FIG. 19 is practiced using the electro-optic combiner 1800 ofFIG. 18 . The method includes an operation 1901 for receiving incominglight through an optical input port (e.g., optical coupler 1805) of thephotonic circuit (e.g., PIC 1801), where a first portion of the incominglight has a first polarization and a second portion of the incominglight has a second polarization. The method also includes an operation1903 for splitting the first portion of the incoming light from thesecond portion of the incoming light. The method also includes anoperation 1905 for directing the first portion of the incoming lightthrough a first optical waveguide (e.g., optical waveguide 1809). Themethod also includes an operation 1907 for rotating the secondpolarization of the second portion of the incoming light to the firstpolarization so that the second portion of the incoming light is apolarization-rotated second portion of the incoming light. The methodalso includes an operation 1909 for directing the polarization-rotatedsecond portion of the incoming light through a second optical waveguide(e.g., optical waveguide 1811). In some embodiments, the operations 1903through 1909 are performed by the PSR 1803. The method also includes anoperation 1911 for operating a plurality of ring resonators (e.g.,1813-1 to 1813-3) disposed between the first optical waveguide and thesecond optical waveguide, where each of the plurality of ring resonatorsis operated to evanescently in-couple light from the first opticalwaveguide and out-couple light into the second optical waveguide. Eachof the plurality of ring resonators is configured to operate at arespective resonant wavelength, such that light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the plurality of ring resonators optically couples from the firstoptical waveguide into the given one of the plurality of ringresonators. The method also includes an operation 1913 for operating aplurality of phase shifters (e.g., 1815-1 to 1815-3) in optical couplingwith the first optical waveguide to apply a controlled amount of shiftto a phase of light traveling through the first optical waveguide. Eachof the plurality of phase shifters is disposed before a respective oneof the plurality of ring resonators with respect to a light propagationdirection through the first optical waveguide. In some embodiments, themethod includes directing light from an output portion of the secondoptical waveguide to one or more photodetectors, where the outputportion of the second optical waveguide is located after the pluralityof ring resonators with respect to the light propagation directionthrough the second optical waveguide.

FIG. 20 shows an example configuration of an electro-optic combiner 2000implemented within a PIC 2001, in accordance with some embodiments. Theelectro-optic combiner 2000 includes a PSR 2003 that has an opticalinput 2003A optically connected to receive incoming light from anoptical coupler 2005, by way of an optical waveguide 2006. In someembodiments, the optical input 2003A of the PSR 2003 is directlyoptically coupled to the optical coupler 2005, such that the opticalwaveguide 2006 is not required. In some embodiments, the optical coupler2005 is implemented as an edge coupler. However, in other embodiments,the optical coupler 2005 is implemented as a vertical grating coupler,or as another type of optical coupling device that provides for opticalcoupling of the PIC 2001 to an optical fiber/waveguide 2007. Incominglight is transmitted from the optical fiber/waveguide 2007 into theoptical coupler 2005, as indicated by arrow 2008. The PSR 2003 has afirst optical output 2003B and a second optical output 2003C. The PSR2003 is configured to direct a first portion of the incoming lighthaving a first polarization (TE or TM) through the first optical output2003B. The PSR 2003 is also configured to rotate a polarization of asecond portion of the incoming light from a second polarization(opposite of the first polarization) to the first polarization. In thismanner, the PSR 2003 turns the second portion of the incoming light intoa polarization-rotated second portion of the incoming light. The PSR2003 is configured to direct the polarization-rotated second portion ofthe incoming light through the second optical output 2003C.Alternatively, in some embodiments, the first portion of the incominglight having a first polarization is transmitted through the secondoptical output 2003C, and the polarization-rotated second portion of theincoming light is transmitted through the first optical output 2003B.

The electro-optic combiner 2000 includes a first optical waveguide 2009optically connected to the first optical output 2003B of the PSR 2003.The electro-optic combiner 2000 also includes a second optical waveguide2011 optically connected to the second optical output 2003C of the PSR2003. The first optical waveguide 2009 and the second optical waveguide2011 are formed of a material through which light can be in-coupled,out-coupled, and guided. Each of the first optical waveguide 2009 andthe second optical waveguide 2011 is formed within a surroundingmaterial that has an optical index of refraction sufficiently differentfrom that of the first optical waveguide 2009 and the second opticalwaveguide 2011, respectively, to enable guiding of light within thefirst optical waveguide 2009 and the second optical waveguide 2011. Insome embodiments, first optical waveguide 2009 and the second opticalwaveguide 2011 are formed of a same material. In some embodiments, thefirst portion of the incoming light is transmitted through the firstoptical output 2003B of the PSR 2003 and into the first opticalwaveguide 2009, and travels along the first optical waveguide 2009, asindicated by arrow 2010. Also, the polarization-rotated second portionof the incoming light is transmitted through the second optical output2003C of the PSR 2003 and into the second optical waveguide 2011, andtravels along the second optical waveguide 2011, as indicated by arrow2012. Alternatively, in some embodiments, the first portion of theincoming light is transmitted through the second optical output 2003C ofthe PSR 2003 and into the second optical waveguide 2011, and travelsalong the second optical waveguide 2011, as indicated by arrow 2012.Also, in these alternative embodiments, the polarization-rotated secondportion of the incoming light is transmitted through the first opticaloutput 2003B of the PSR 2003 and into the first optical waveguide 2009,and travels along the first optical waveguide 2009 as indicated by arrow2010.

The electro-optic combiner 2000 includes a first plurality of ringresonators 2013-1 to 2013-3 positioned along the first optical waveguide2010 and within an evanescent optical coupling distance of the firstoptical waveguide 2010. The electro-optic combiner 2000 also includes asecond plurality of ring resonators 2017-1 to 2017-3 positioned alongthe second optical waveguide 2011 and within an evanescent opticalcoupling distance of the second optical waveguide 2011. The firstplurality of ring resonators 2013-1 to 2013-3 and the second pluralityof ring resonators 2017-1 to 2017-3 are positioned between the firstoptical waveguide 2009 and the second optical waveguide 2011. Each ofthe second plurality of ring resonators 2017-1 to 2017-3 is positionedto optically in-couple light from a respective one of the firstplurality of ring resonators 2013-1 to 2013-3. While the exampleelectro-optic combiner 2000 shows three ring resonators 2013-1 to 2013-3and three ring resonators 2017-1 to 2017-3 for purposes of description,it should be understood that there is no limit on the number of thesering resonators, so long as the ring resonators and associated signalprocessing circuitry can be spatially and electrically accommodated onthe chip. Also, a number of the second plurality of ring resonators2017-1 to 2017-3 is equal to a number of the first plurality of ringresonators 2013-1 to 2013-3, such that the first plurality of ringresonators 2013-1 to 2013-3 and the second plurality of ring resonators2017-1 to 2017-3 collectively form a plurality of pairs of ringresonators, where each ring resonator within a given pair of ringresonators is operated at a same resonant wavelength. Each pair of ringresonators 2013-1/2017-1 to 2013-3/2017-3 is functions as a double-ringfilter. The ring resonance wavelength of the double ring filter can betuned relative to the channel wavelength to account for the phase andintensity imbalance of the first optical waveguide 2009 and the secondoptical waveguide 2011 over time.

In some embodiments, the ring resonators 2013-1 to 2013-3 and 2017-1 to2017-3 are implemented as annular-shaped waveguides having circuitousconfiguration, e.g., circular, oval, race-track, or another arbitrarycircuitous shape. In some embodiments, the ring resonators 2013-1 to2013-3 and 2017-1 to 2017-3 are implemented as circular discs. The ringresonators 2013-1 to 2013-3 and 2017-1 to 2017-3 are formed of amaterial through which light can be in-coupled, out-coupled, and guided.Each of the ring resonators 2013-1 to 2013-3 and 2017-1 to 2017-3 isformed within a surrounding material that has an optical index ofrefraction sufficiently different from that of the ring resonators2013-1 to 2013-3 and 2017-1 to 2017-3 to enable guiding of light withinthe ring resonators 2013-1 to 2013-3 and 2017-1 to 2017-3 and around thecircuitous path defined by each of the ring resonators 2013-1 to 2013-3and 2017-1 to 2017-3. In some embodiments, each of the ring resonators2013-1 to 2013-3 and 2017-1 to 2017-3 is configured to have anannular-shape or disc-shape with an outer diameter of less than about 50micrometers. In some embodiments, each of the ring resonators 2013-1 to2013-3 and 2017-1 to 2017-3 is configured to have an annular-shape ordisc-shape with an outer diameter of less than about 10 micrometers.

The first plurality of ring resonators 2013-1 to 2013-3 in-couples lightfrom the first optical waveguide 2009 and out-couples light intorespective ones of the second plurality of ring resonators 2017-1 to2017-3. The second plurality of ring resonators 2017-1 to 2017-3in-couples light from respective ones of the first plurality of ringresonators 2013-1 to 2013-3 and out-couples light into the secondoptical waveguide 2011. In this manner, the first plurality of ringresonators 2013-1 to 2013-3 and the second plurality of ring resonators2017-1 to 2017-3 collectively operate to couple light from the firstoptical waveguide 2009 to the second optical waveguide 2011, asindicated by arrows 2014. A light propagation direction through thefirst plurality of ring resonators 2013-1 to 2013-3 is opposite of alight propagation direction through the second plurality of ringresonators 2017-1 to 2017-3. In the example electro-optic combiner 2000,light propagates in a counter-clockwise direction, as indicated byarrows 2016-1 to 2016-3, within each of the first plurality of ringresonators 2013-1 to 2013-3, and light propagates in a clockwisedirection, as indicated by arrows 2018-1 to 2018-3, within each of thesecond plurality of ring resonators 2017-1 to 2017-3.

Each of the first plurality of ring resonators 2013-1 to 2013-3 isconfigured to operate at a respective resonant wavelength (λ₁ to λ₃),such that light having a wavelength substantially equal to therespective resonant wavelength of a given one of the first plurality ofring resonators 2013-1 to 2013-3 optically couples from the firstoptical waveguide 2009 into the given one of the plurality of ringresonators 2013-1 to 2013-3. Each of the second plurality of ringresonators 2017-1 to 2017-3 is configured to operate at a respectiveresonant wavelength (λ₁ to λ₃), such that light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the second plurality of ring resonators 2017-1 to 2017-3 opticallycouples from the corresponding one of the first plurality of ringresonators 2013-1 to 2013-3 into the given one of the second pluralityof ring resonators 2017-1 to 2017-3. The light that is coupled into thesecond plurality of ring resonators 2017-1 to 2017-3 travels around thesecond plurality of ring resonators 2017-1 to 2017-3 in a clockwisedirection and couples into the second optical waveguide 2011, asindicated by arrows 2014. In this manner, the light output from the PSR2003 into the first optical waveguide 2009 is combined with the lightoutput by the PSR 2003 into the second optical waveguide 2011. Thecombined light within the second optical waveguide 2011 after the secondplurality of ring resonators 2017-1 to 2017-3 (with respect to the lightpropagations directions 2012 and 2014) is output from the electro-opticcombiner 2000. In some embodiments, the combined light is output fromthe electro-optic combiner 2000 to photodetectors. However, in otherembodiments, the combined light is output from the electro-opticcombiner 2000 to essentially any type of photonic device, as needed. Thering resonators 2013-1 to 2013-3 and 2017-1 to 2017-3 function aspassive filters to combine the light signals output by the PSR 2003.

In some embodiments, the first optical waveguide 2009 includes a firstsection 2009A extending from the first optical output 2003B of the PSR2003 to a nearest one (2013-1) of the first plurality of ring resonators2013-1 to 2013-3 to the PSR 2003. Also, the second optical waveguide2011 includes a first section 2011A extending from the second opticaloutput 2003C of the PSR 2003 to a nearest one (2017-1) of the secondplurality of ring resonators 2017-1 to 2017-3 to the PSR 2003. In theseembodiments, either the first section 2009A of the first opticalwaveguide 2009 is longer than the first section 2011A of the secondoptical waveguide 2011, or the first section 2011A of the second opticalwaveguide 2011 is longer than the first section 2009A of the firstoptical waveguide 2009, in order to compensate for a timing delaybetween the first portion of the incoming light exiting the PSR 2003 andthe polarization-rotated second portion of the incoming light exitingthe PSR 2003, so as to minimize a timing-skew (timing difference)between transmission of the first portion of the incoming light into thefirst optical waveguide 2009 and transmission of thepolarization-rotated second portion of the incoming light into thesecond optical waveguide 2011. In the example electro-optic combiner2000, the first section 2009A of the first optical waveguide 2009includes a delay section 2009B configured so that the optical pathlength through the first section 2009A of the first optical waveguide2009 is longer than the optical path length through the first section2011A of the second optical waveguide 2011. The delay section 2009B isconfigured to compensate for the timing delay between the first portionof the incoming light exiting the PSR 2003 and the polarization-rotatedsecond portion of the incoming light exiting the PSR 2003. The delaysection 2009B is configured to ensure broadband operation of theelectro-optic combiner 2000. The delay section 2009B is designed tocompensate for the differential group delay between the twopolarizations accumulated when propagating through PIC 2001 componentssuch as the optical coupler 2005, the PSR 2003, and routing waveguides2006, 2009, 2011. In the absence of the delay section 2009B, a pluralityof independent phase shifters can be optically coupled to the firstoptical waveguide 2009, such that one of the plurality of independentphase shifters is positioned before a respective one of the firstplurality of ring resonators 2013-1 to 2013-3 (similar to the phaseshifters 1815-1 to 1815-3 described with regard to FIG. 18 ).

With the delay section 2009B provided with the first section 2009A ofthe first optical waveguide 2009, the electro-optic combiner 2000 isable to implement a single phase shifter 2019 on either the firstoptical waveguide 2009 or the second optical waveguide 2011 at aposition before the ring resonator pairs 2013-1/2017-1 to 2013-3/2017-3.In the example electro-optic combiner 2000, the phase shifter 2019 isimplemented on the first optical waveguide 2009 before the first ringresonator 2013-1 of the first plurality of ring resonators 2013-1 to2013-3. The phase shifter 2019 is tuned along with the resonancewavelength of each of the ring resonator pairs 2013-1/2017-1 to2013-3/2017-3, relative to the wavelength of the channel of the incominglight signal to which the ring resonator pair couples, to ensurelow-loss combining of the optical signals into the second opticalwaveguide 2011 (the output waveguide). In some embodiments, the phaseshifter 2019 is implemented as a thermal tuner (e.g., heating device)positioned over the first optical waveguide 2009, which operates byexploiting the thermo-optic effect of the first optical waveguide 2009material. In some embodiments, the phase shifter 2019 is implemented asan electro-optic device (e.g., diode) built into the first opticalwaveguide 2009, which operates by exploiting electro-optic effectswithin the first optical waveguide 2009. In some embodiments, the phaseshifter 2019 is implemented as a set of ring resonators.

FIG. 21 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments. In some embodiments, themethod of FIG. 21 is practiced using the electro-optic combiner 2000 ofFIG. 20 . The method includes an operation 2101 for receiving incominglight through an optical input port (e.g., optical coupler 2005) of thephotonic circuit (e.g., PIC 2001), where a first portion of the incominglight has a first polarization and a second portion of the incominglight has a second polarization. The method includes an operation 2103for splitting the first portion of the incoming light from the secondportion of the incoming light. The method includes an operation 2105 fordirecting the first portion of the incoming light through a firstoptical waveguide (e.g., optical waveguide 2009). The method includes anoperation 2107 for rotating the second polarization of the secondportion of the incoming light to the first polarization so that thesecond portion of the incoming light is a polarization-rotated secondportion of the incoming light. The method includes an operation 2109 fordirecting the polarization-rotated second portion of the incoming lightthrough a second optical waveguide (e.g., optical waveguide 2011). Insome embodiments, the operations 2103 through 2109 are performed by thePSR 2003. The method includes an operation 2111 for operating a firstplurality of ring resonators (e.g., 2013-1 to 2013-3) disposed betweenthe first optical waveguide and the second optical waveguide, where eachof the first plurality of ring resonators is operated to evanescentlyin-couple light at a particular channel wavelength from the firstoptical waveguide. The method includes an operation 2113 for operating asecond plurality of ring resonators (e.g., 2017-1 to 2017-3) disposedbetween the first optical waveguide and the second optical waveguide,where each of the second plurality of ring resonators is operated toevanescently in-couple light at a particular channel wavelength from arespective one of the first plurality of ring resonators. Each of thesecond plurality of ring resonators is also operated to evanescentlyout-couple light to the second optical waveguide. Each optically coupledpair of ring resonators within the first plurality of ring resonatorsand the second plurality of ring resonators is operated at asubstantially same resonant wavelength. Also, each optically coupledpair of ring resonators within the first plurality of ring resonatorsand the second plurality of ring resonators has an opposite lightpropagation direction. In some embodiments, the method includesoperating a phase shifter (e.g., phase shifter 2019) in optical couplingwith the first optical waveguide to apply a controlled amount of shiftto a phase of light traveling through the first optical waveguide. Also,in some embodiments, the method also includes routing light from aoutput section of the second optical waveguide to one or morephotodetectors, wherein the output section of the second opticalwaveguide is located after the second plurality of ring resonators withrespect to a light propagation direction through the second opticalwaveguide.

FIG. 22 shows an example configuration of an electro-optic combiner 2200implemented within a PIC 2201, in accordance with some embodiments. Theelectro-optic combiner 2200 is a modification of the electro-opticcombiner 2000 of FIG. 20 . Specifically, the electro-optic combiner 2200includes all of the components of the electro-optic combiner 2000, andfurther includes a plurality of intermediate optical waveguides 2203-1to 2203-3 respectively disposed between the first plurality of ringresonators 2013-1 to 2013-3 and the second plurality of ring resonators2017-1 to 2017-3. Each of the plurality of intermediate opticalwaveguides 2203-1 to 2203-3 is positioned between a corresponding one ofthe first plurality of ring resonators 2013-1 to 2013-3 configured tooperate at a specified resonant wavelength and a corresponding one ofthe second plurality of ring resonators 2017-1 to 2017-3 configured tooperate at the same specified resonant wavelength. Light having thespecified resonant wavelength optically couples from the first opticalwaveguide 2009 to the corresponding one of the first plurality of ringresonators 2013-1 to 2013-3, and from the corresponding one of the firstplurality of ring resonators 2013-1 to 2013-3 to the corresponding oneof the plurality of intermediate optical waveguide 2203-1, and from theintermediate optical waveguide 2203-1 to 2203-3 to the corresponding oneof the second plurality of ring resonators 2017-1 to 2017-3, and fromthe corresponding one of the second plurality of ring resonators 2017-1to 2017-3 to the second optical waveguide 2011. In some embodiments,each of the plurality of intermediate optical waveguides 2203-1 to2203-3 has a substantially linear shape and is oriented to have asubstantially same lengthwise direction of extent. In some embodiments,such as shown in FIG. 22 , the first plurality of ring resonators 2013-1to 2013-3 and the second plurality of ring resonators 2017-1 to 2017-3are offset with respect to each other in a direction substantiallyparallel to the lengthwise direction of extent of the plurality ofintermediate optical waveguides 2203-1 to 2203-3.

The electro-optic combiner 2200 also includes a plurality ofphotodetectors 2205-1 to 2205-3 respectively optically connected to theplurality of intermediate optical waveguides 2203-1 to 2203-3, such thatsome of the light that optically couples into a given one of theplurality of intermediate optical waveguides 2203-1 to 2203-3 from thecorresponding one of the first plurality of ring resonators 2013-1 to2013-3 is conveyed into one of the plurality of photodetectors 2205-1 to2205-3 that is optically connected to the given one of the plurality ofintermediate optical waveguides 2203-1 to 2203-3. In some embodiments,the electro-optic combiner 2200 includes feedback circuitry 2207configured to control the resonant wavelengths of the first plurality ofring resonators 2013-1 to 2013-3 and the second plurality of ringresonators 2017-1 to 2017-3 using electrical signals (photocurrentsignals) output from corresponding ones of the plurality ofphotodetectors 2205-1 to 2205-3. Also, in some embodiments, the feedbackcircuitry 2207 is configured to control the phase shifter 2019 usingelectrical signals output from the plurality of photodetectors 2205-1 to2205-3. The plurality of intermediate optical waveguides 2203-1 to2203-3 advantageously provide for better control over the evanescentoptical coupling between the first plurality of ring resonators 2013-1to 2013-3 and respective ones of the second plurality of ring resonators2017-1 to 2017-3. The plurality of intermediate optical waveguides2203-1 to 2203-3 also advantageously provide a linear optical tap tofeed into a feedback control system (feedback circuitry 2207) for thefirst plurality of ring resonators 2013-1 to 2013-3, the secondplurality of ring resonators 2017-1 to 2017-3, and the phase shifter2019. In some embodiments, the optimal tuning of the first plurality ofring resonators 2013-1 to 2013-3 and the second plurality of ringresonators 2017-1 to 2017-3, and the optimum phase shift in the firstoptical waveguide 2009 by the phase shifter 2019 will result in aminimum amount of optical power entering each of the plurality ofphotodetectors 2205-1 to 2205-3, which allows a control system toseparately and independently optimize the output transmission for eachwavelength channel in the incoming light signal.

FIG. 23 shows a flowchart of a method for combination of opticalsignals, in accordance with some embodiments. In some embodiments, themethod of FIG. 23 is practiced using the electro-optic combiner 2200 ofFIG. 22 . The method includes an operation 2301 for receiving incominglight through an optical input port (e.g., optical coupler 2005) of thephotonic circuit (e.g., PIC 2001), where a first portion of the incominglight has a first polarization and a second portion of the incominglight has a second polarization. The method includes an operation 2303for splitting the first portion of the incoming light from the secondportion of the incoming light. The method includes an operation 2305 fordirecting the first portion of the incoming light through a firstoptical waveguide (e.g., optical waveguide 2009). The method includes anoperation 2307 for rotating the second polarization of the secondportion of the incoming light to the first polarization so that thesecond portion of the incoming light is a polarization-rotated secondportion of the incoming light. The method includes an operation 2309 fordirecting the polarization-rotated second portion of the incoming lightthrough a second optical waveguide (e.g., optical waveguide 2011). Insome embodiments, the operations 2303 through 2309 are performed by thePSR 2003. The method includes an operation 2311 for operating a firstplurality of ring resonators (e.g., 2013-1 to 2013-3) disposed betweenthe first optical waveguide and the second optical waveguide, where eachof the first plurality of ring resonators is operated to evanescentlyin-couple light at a particular channel wavelength from the firstoptical waveguide.

The method also includes an operation 2313 for optically coupling lightfrom each of the first plurality of ring resonators into a correspondingone of a plurality of intermediate optical waveguides (e.g., opticalwaveguides 2203-1 to 2203-3). The method includes an operation 2315 foroperating a second plurality of ring resonators (e.g., 2017-1 to 2017-3)disposed between the first optical waveguide and the second opticalwaveguide, where each of the second plurality of ring resonators isoperated to evanescently in-couple light at a particular channelwavelength from a respective one of the plurality of intermediateoptical waveguides. Each of the second plurality of ring resonators isalso operated to evanescently out-couple light to the second opticalwaveguide. Each optically connected pair of ring resonators within thefirst plurality of ring resonators and the second plurality of ringresonators is operated at a substantially same resonant wavelength.Also, each optically connected pair of ring resonators within the firstplurality of ring resonators and the second plurality of ring resonatorshas an opposite light propagation direction. In some embodiments, themethod includes operating a phase shifter (e.g., phase shifter 2019) inoptical coupling with the first optical waveguide to apply a controlledamount of shift to a phase of light traveling through the first opticalwaveguide. Also, in some embodiments, the method includes routing lightfrom a output section of the second optical waveguide to one or morephotodetectors, wherein the output section of the second opticalwaveguide is located after the second plurality of ring resonators withrespect to a light propagation direction through the second opticalwaveguide.

In some embodiments, the method includes operating a plurality ofphotodetectors (e.g., photodetectors 2205-1 to 2205-3) to detect anamount light optically coupled into a respective ones of the pluralityof intermediate optical waveguides. In some embodiments, the methodincludes controlling resonant wavelengths of the first plurality of ringresonators and the second plurality of ring resonators in accordancewith photocurrents generated by corresponding ones of the plurality ofphotodetectors to optimize an amount of optical power conveyed from thefirst optical waveguide to the second optical waveguide by way of thefirst plurality of ring resonators, the plurality of intermediatewaveguides, and the second plurality of ring resonators. In someembodiments, the method includes controlling operation of the phaseshifter in accordance with photocurrents generated by the plurality ofphotodetectors.

FIG. 24A shows a diagram of an electro-optic receiver that is configuredto tolerate polarization-dependent timing-skew, in accordance with someembodiments. The electro-optic receiver accepts photocurrent from aphotodetector and performs amplification and linear equalization, asindicated by block 2401. In some embodiments, the linear equalization isused to cancel added ISI due to polarization skew. Next, the filteredand amplified signal is sampled to extract the data, as indicated byblock 2403. Also, in some embodiments, the filtered and amplified signalundergoes non-linear equalization such as decision feedback equalization(DFE). Simultaneously, the precursor ISI, the postcursor ISI, and themain tap height are measured through a data level slicer (dLev). Also, aclock-data recovery (CDR) samples the filtered data and outputs data toextract the optimal sampling time, as indicated by block 2405.Information about the ISI is sent to the equalization (EQ) adaptationblock to analyze the residual ISI and adjust the filter weightsaccordingly, as indicated by block 2407.

FIG. 24B shows a modification of the electro-optic receiver of FIG. 24A,in accordance with some embodiments. In this embodiment, informationabout the ISI is also sent to a polarization detector, as indicated byblock 2409. Based on the relative strength of the precursor ISI and thepostcursor ISI, information about the polarization state is extractedand is then used to adjust the EQ adaptation and CDR to move to an ideallock position.

FIG. 24C shows a modification of the electro-optic receiver of FIG. 24B,in accordance with some embodiments. In this embodiment, the linearphotodetector 400 of FIG. 4 is implemented to output photocurrents fromeach of the two polarizations (TE and TM) to separate outputs formeasurement by the polarization detector, as indicated by block 2409.The polarization detector provides a reverse bias to the linearphotodetector 400, and measures the amount of photocurrent flowingthrough each bias. By comparing the relative photocurrent of each bias,information about the polarization state is extracted. This informationis then used to adjust the EQ adaptation and CDR to move to the ideallock position.

In some embodiments, the electro-optic receiver includes equalizationand timing recovery circuits to handle the combined effects ofpolarization-dependent timing-skew, bandwidth limitations, and clocktiming jitter. In some embodiments, the electro-optic receiver includesamplifiers for the purpose of conditioning the signal for equalizationand clock-data recovery. In some embodiments, the electro-optic receiverincludes an adaptive equalization circuit which detects the presence ofISI, and corrects for it with linear filters or non-linear feedbackcontrol through DFE. In some embodiments, the amount of ISI is measuredduring operation by a monitor or dLev. Information about the relativeISI tap weights is used to adapt the equalization circuit to minimizethe residual ISI. In some embodiments, the electro-optic receiverincludes CDR circuitry which detects the optimal time to sample theincoming signal by extracting timing information from data transitionsand adjusting the internal sampling clock accordingly.

In some embodiments, the input polarization may not be well controlled,and can vary over time. The presence of a time-varying inputpolarization results in time-varying ISI conditions. Time-varying inputpolarization can also dynamically shift the electro-optic receiver'soptimal sampling time by up to the skew between the two polarizationsduring operation. In some embodiments, without further correction, theCDR and equalization circuitry may encounter conditions where it willnot lock to the optimal settings. In some embodiments, the electro-opticreceiver contains additional circuitry to detect the relative split inoptical power between polarization states during operation. In someembodiments, the spatial distribution of photocurrent generation in thephotodetector may be used to measure the polarization state of theinput. In some embodiments, a linear photodetector (e.g., 400) may beused where light is input from two different sides, where each sidesupplies light from one input polarization. In these embodiments, theintensity of light, and as a result the generated carriers, from onepolarization decays exponentially across the length of the photodetectoraccording to the photo-absorption coefficient. Due to this, a majorityof light from one polarization is absorbed on one half of thephotodetector, and a majority of light from the other polarization isabsorbed on the other half of the photodetector. In these embodiments,the contacts to the photodetector may be segmented and connected to aplurality of different reverse biasing and receiver circuits. Bycomparing the relative photocurrent measured between differentreceivers, the relative power split between different polarizations canbe determined.

In some embodiments changes to the relative amount of ISI can be used todetect the polarization state of the input. The impact of polarizationtiming-skew on ISI is minimized when the input polarization directlyaligns with one of the linear polarizations of the receive chip. Theimpact of polarization timing-skew on ISI is maximized when the inputpolarization splits power evenly between the two linear polarizations ofthe receive chip. In some embodiments, dLev may be used to measure themagnitude of ISI during operation. Information about changes in themagnitude of ISI, and changes to the ratio of the precursor ISI and thepostcursor ISI can be used to infer shifts in the polarization state.Also, in some embodiments, information about the polarization state canbe used to dynamically adjust the CDR circuitry to cancel drift in theoptimal sampling position. In some embodiments, information about thepolarization state can be used to dynamically adjust the equalizationcircuitry to minimize residual ISI. In some embodiments, informationabout the polarization state can be used to detect and separate datastreams that have been combined using polarization multiplexing.

In some embodiments, the optical coupler that couples light from theinput optical fiber/waveguide into the PIC is a dual-polarizationvertical grating coupler that routes light from different inputpolarizations directly into two output waveguides on the PIC, possiblywith the same waveguide polarization. In some embodiments, the lightfrom the two polarizations of the input optical fiber/waveguide is splitinto separate directions by a polarization beam splitter, and is coupledinto two separate PIC waveguides through two separate vertical gratingcouplers, or through two separate edge couplers, or through any othercoupling scheme. In various embodiments, the polarization beam-splitteris a separate device, or is built into the input optical fiber/waveguideitself by a suitable modification of the input optical fiber/waveguidetermination.

In some embodiments, the light from the two polarizations are input intothe same waveguide of the PIC, with different waveguide polarizations,either through edge-coupling or vertical grating coupling or some othermethod, and the splitting of the signal into two different waveguides isdone by a polarization splitter built into the PIC. In such cases, theoutput of an integrated polarization splitter provides two outputwaveguides each carrying one polarization mode, and the polarizationmodes are different, such that one polarization mode is TE-like and theother polarization mode is TM-like. Hence, in some embodiments, afurther integrated polarization rotator converts one of the output modesto match the same polarization state as the other, in a matchingwaveguide cross-section. In other embodiments, the two outputs of thepolarization splitter device contain a combination of two inputorthogonal polarization states. For example, the two outputs of thepolarization splitter device contain the sum and the difference of theinput TM and TE waves.

In some embodiments, the PIC is built on a semiconductor chip, such as asilicon or indium phosphide based chip. In some embodiments, theelectronics of the electro-optic receiver are co-located with theoptical devices on the chip. In some embodiments, the electrical signalfrom the photodetector is routed off-chip to an external receivercircuit. In some embodiments, the PIC is built out of glass, and the twowaveguides in the glass PIC receiving the input optical fiber/waveguidesignal are routed to a photodetector on a different chip, either throughbutt-coupling of two chips, or through a connecting external opticalfiber/waveguide.

In some embodiments, WDM is used to receive information from differentwavelength channels within the input optical fiber/waveguide. In theseembodiments, the PIC will have a plurality of photodetectors, with eachphotodetector detecting a single wavelength channel within a narrow,distinct wavelength range. In some embodiments, all of a plurality ofphotodetectors are placed near a single bus optical waveguide thatconnects the two PIC waveguides receiving the split input signal. Thephotodetectors are designed to couple to the optical signal in thesingle bus optical waveguide, from either direction, only if the opticalsignal falls within the specified wavelength range of a givenphotodetector, which allows multiple photodetectors to operateindependently on the single (shared) bus optical waveguide. In someembodiments, the plurality of photodetectors are built into ringresonators, or disk resonators, or other resonant photodetectors withwavelength selectivity control. In some embodiments, the plurality ofphotodetectors are linear detectors. In some embodiments, passive ringresonators are used as WDM filters, passing each wavelength channel ofthe incoming optical signal to a single linear photodetector thatdetects only data from a single wavelength channel.

In some embodiments in which the photodetector is a resonant device,such as a ring resonator or a Fabry-Perot resonator, or a non-resonantlinear photodetector, a standing wave or partial standing wave will formwithin the photodetector if it receives light signals from two oppositedirections. This standing wave pattern will manifest itself as an arrayof discrete positions within the photodetector where the optical poweris high. In some configurations, the photodetector is built in such away that the responsivity varies locally within the photodetectorcavity. For example, in some embodiments, the photodetector includes aset of interleaved diodes formed by non-uniform dopant profilesthroughout the photodetector cavity. In another example, in someembodiments, the photodetector includes a division of thephoto-absorptive material into “islands” of discrete areas. In someembodiments, there is a chance that the standing wave will isolate theoptical power density into discrete parts of the photodetector that donot have strong responsivity. In some embodiments, to address thisissue, the photodetectors are configured so that the averageresponsivity over the length of the photodetector is not minimized. Forexample, in some embodiments, the photodetector is configured to haveappropriate spacing or placement of the photodetector regions that showhigh responsivity, so as to spatially align with the peak amplitudelocations of the standing wave within the photodetector cavity.

In some embodiments, a differential delay to the photodetector via thetwo PIC waveguides carrying the two components of the polarization stateis compensated with an optical circuit. For example, in theelectro-optic receiver 300 of FIG. 3 , optical delay lines areimplemented to equalize the waveguide path length for each wavelengthchannel of the two split signals, thereby reducing the time delay of thesignals to the photodetector. In some cases, this eliminates the needfor a receiver timing-skew management system. In other cases, theuncertainty in the timing delay still necessitates a receivertiming-skew management system, but the optical delay lines will enablethe size of the system (such as the spacing between the detectors) to bemuch larger, which helps to accommodate the size of the receiver andother circuitry in embodiments where the photodetectors and receivercircuits are co-located on the same chip, and which helps to reducepackaging constraints for embodiments where the receiver circuits arelocated on a separate chip.

In some embodiments, an additional PIC optical skew compensator (OSC) isinserted between the inputs of the two PIC waveguides carrying the twocomponents of the polarization state and the plurality of channelreceivers. The OSC is designed to provide a group delay as a function offrequency (wavelength) to match the group delay mismatch imposed by thewaveguide length for the wavelength channels. In some embodiments, theOSC provides a linear group delay with frequency, and the channelreceivers are arranged along the receiver waveguide in order ofmonotonically increasing frequency. In some embodiments, two OSC's areprovided, each providing half of the timing-skew delay compensation,with one OSC at each of the two PIC waveguide inputs. In someembodiments, the two OSC's provide linearly ramped group delay withfrequency, with the first OSC having an increasing ramped group delaywith frequency, and with the second OSC having a decreasing ramped groupdelay with frequency. In some embodiments, the OSC includes an all passfilter. In some embodiments, the OSC includes a set of ring (microring)resonator all-pass filters. In some embodiments, the OSC providesminimal insertion loss at all wavelengths corresponding to the WDMchannels, and tailored group delays. In some embodiments, the OSC'sslope of group delay ramp with increasing frequency is such that, over asingle channel spacing, the group delay difference produced isapproximately equal to the group delay difference produced by thedifference in physical position of the channel receivers for twoadjacent channels.

In some embodiments disclosed herein, a channel receiver circuit isprovided that is capable of compensating for the two-path differentialgroup delay. Also, in some embodiments disclosed herein, a WDM receiverarchitecture is provided as a wrap-around loop. Also, in someembodiments disclosed herein, a channel receiver photodiode is providedthat avoids issues with arbitrarily distributed input light between twoinput ports (e.g., interdigitated photodetector with number of junctionsdifferent from number of wavelengths around at operating wavelength—tohave constant responsivity versus detuning, with no nulls). Also, insome embodiments disclosed herein, a method and a system are providedfor combining polarizations into a single mode with a feedback control,including use of an integrated delay line for timing-skew compensationto ensure broadband operation, which is generalized to WDMcommunication. Also, in some embodiments disclosed herein, a transmitterwith modulation of orthogonal polarization components with the same bitpattern is provided.

In some embodiments disclosed herein, an electro-optic receiverconfiguration is provided in which an optical signal having arbitrarypolarization is coupled from an input optical fiber/waveguide into a PICand is detected by one or more optical detectors within the PIC,regardless of the polarization state of the light of the optical signalin the input optical fiber/waveguide. In some embodiments, theelectro-optic receiver includes a polarization beam-splitting androtating device that couples light having uncontrolled polarization fromthe input optical fiber/waveguide into two separate ends of a sameloop-structured optical waveguide within the PIC, such that light fromeach linear polarization is coupled into a different end of the sameloop-structured optical waveguide within the PIC, and such that lighthaving one of the two linear polarizations is rotated to the otherpolarization before coupling into the loop-structured optical waveguide.In this manner, light propagating through the loop-structured opticalwaveguide from either the first end or the second end has the samepolarization and can be detected by the same photodetector. Thiseliminates the need to have duplicate photodetector devices fordetecting the two polarizations of light, respectively, therebyoptimizing chip area usage and reducing cost.

Also, in some embodiments, the electro-optic receiver includes apolarization beam-splitting and rotating device that couples lighthaving uncontrolled polarization from the input optical fiber/waveguideinto two separate waveguides within the PIC, such that light from eachlinear polarization within the input optical fiber/waveguide is coupledinto a separate waveguide within the PIC. Also, the polarizationbeam-splitting and rotating device is designed to rotate a polarizationof light having one of the two linear polarizations to the otherpolarization before coupling of the light into the respective waveguidewithin the PIC. In this manner, light coupled into the two waveguideswithin the PIC have the same preferred waveguide polarization. The twowaveguides within the PIC are routed to the same photodetector device(or set of photodetector devices), which allows any polarization of theinput optical signal to be detected in the same photodetector device.This eliminates the need to have duplicate photodetector devices fordetecting the two polarizations of light, respectively, therebyoptimizing chip area usage and reducing cost.

In some embodiments, in the event that the optical signal from the inputoptical fiber/waveguide contains both polarizations, the light of theoptical signal will be coupled into both waveguides of the PIC. Sincethe two waveguides within the PIC have different lengths to reach agiven photodetector device, each polarization component of the opticalsignal may reach the given photodetector device at different times, soas to have a timing difference (timing-skew). In such cases, thetiming-skew management system is implemented in the electro-opticreceiver circuitry to enable the electro-optic receiver to faithfullyrecover the optical signal as received from the input opticalfiber/waveguide even with the timing difference. In cases where thetiming difference is too large to be handled by the timing-skewmanagement system, an optical delay line is implemented within the PICto either reduce the timing difference to a low enough level that can behandled by the timing-skew management system, or eliminate the timingdifference.

It should be appreciated that the electro-optic receiver embodimentsdisclosed herein are useful in applications where electro-opticreceivers detect light from an input optical fiber/waveguide in whichthe polarization is not controlled. A polarization beam-splitter androtator, such as the dual-polarization grating coupler among others, isused in some embodiments to transmit incoming light of eitherpolarization into a preferred polarization of the electro-optic receiverPIC. In some embodiments, with the input optical signal split into twoseparate optical waveguides within the PIC, the light from the twooptical waveguides cannot be combined into a single waveguide in alow-loss, broadband way without complex phase control and optical powermonitoring systems. To mitigate this issue, various embodiments of theelectro-optic receiver disclosed herein provide for non-simultaneousdetection of the polarization-split input optical signal by the samedetector or set of detectors, thereby reducing cost and complexity ofthe electro-optic receiver.

The foregoing description of the embodiments has been provided forpurposes of illustration and description, and is not intended to beexhaustive or limiting. Individual elements or features of a particularembodiment are generally not limited to that particular embodiment, but,where applicable, are interchangeable and can be used in a selectedembodiment, even if not specifically shown or described. In this manner,one or more features from one or more embodiments disclosed herein canbe combined with one or more features from one or more other embodimentsdisclosed herein to form another embodiment that is not explicitlydisclosed herein, but rather that is implicitly disclosed herein. Thisother embodiment may also be varied in many ways. Such embodimentvariations are not to be regarded as a departure from the disclosureherein, and all such embodiment variations and modifications areintended to be included within the scope of the disclosure providedherein.

Although some method operations may be described in a specific orderherein, it should be understood that other housekeeping operations maybe performed in between method operations, and/or method operations maybe adjusted so that they occur at slightly different times orsimultaneously or may be distributed in a system which allows theoccurrence of the processing operations at various intervals associatedwith the processing, as long as the processing of the method operationsare performed in a manner that provides for successful implementation ofthe method.

Although the foregoing embodiments have been described in some detailfor purposes of clarity of understanding, it will be apparent thatcertain changes and modifications can be practiced within the scope ofthe appended claims. Accordingly, the embodiments disclosed herein areto be considered as illustrative and not restrictive, and are thereforenot to be limited to just the details given herein, but may be modifiedwithin the scope and equivalents of the appended claims.

What is claimed is:
 1. An electro-optic receiver, comprising: apolarization splitter and rotator having an optical input opticallyconnected to receive incoming light, the polarization splitter androtator having a first optical output and a second optical output, thepolarization splitter and rotator configured to direct a first portionof the incoming light having a first polarization through the firstoptical output, the polarization splitter and rotator configured torotate a polarization of a second portion of the incoming light from asecond polarization to the first polarization so that the second portionof the incoming light is a polarization-rotated second portion of theincoming light, the polarization splitter and rotator configured todirect the polarization-rotated second portion of the incoming lightthrough the second optical output; an optical waveguide having a firstend optically to the first optical output of the polarization splitterand rotator, the optical waveguide having a second end opticallyconnected to the second optical output of the polarization splitter androtator, such that the first portion of the incoming light travels fromthe first optical output of the polarization splitter and rotatorthrough the optical waveguide in a first direction, and such that thepolarization-rotated second portion of the incoming light travels fromthe second optical output of the polarization splitter and rotatorthrough the optical waveguide in a second direction opposite the firstdirection; a plurality of ring resonators positioned alongside theoptical waveguide and within an evanescent optical coupling distance ofthe optical waveguide, each of the plurality of ring resonatorsconfigured to operate at a respective resonant wavelength, such that thefirst portion of the incoming light having a wavelength substantiallyequal to the respective resonant wavelength of a given one of theplurality of ring resonators optically couples into the given one of theplurality of ring resonators in a first propagation direction, and suchthat the polarization-rotated second portion of the incoming lighthaving a wavelength substantially equal to the respective resonantwavelength of the given one of the plurality of ring resonatorsoptically couples into the given one of the plurality of ring resonatorsin a second propagation direction opposite the first propagationdirection; a plurality of photodetectors respectively associated withthe plurality of ring resonators; and a plurality of output opticalwaveguides respectively optically coupled to the plurality of ringresonators, each of the plurality of output optical waveguides includinga coupling section, a short section, and a long section, the couplingsection positioned to evanescently couple light from a corresponding oneof the plurality of ring resonators, the short section extending from afirst end of the coupling section to a corresponding one of theplurality of photodetectors, the long section extending from a secondend of the coupling section to the corresponding one of the plurality ofphotodetectors.
 2. The electro-optic receiver as recited in claim 1,wherein a length of the long section and a length of the short sectionare defined to reduce a difference in arrival time of the first portionof the incoming light and the polarization-rotated second portion of theincoming light at the corresponding one of the photodetectors to whichthe long section and the short section are optically connected.
 3. Theelectro-optic receiver as recited in claim 1, wherein the length of thelong section is different for each of plurality of output opticalwaveguides.
 4. The electro-optic receiver as recited in claim 1, whereinthe length of the long section decreases as a distance between thecorresponding one of the plurality of ring resonators and a midpoint ofthe optical waveguide decreases, wherein the midpoint of the opticalwaveguide is about halfway between the first end of the opticalwaveguide and the second end of the optical waveguide.
 5. Theelectro-optic receiver as recited in claim 1, wherein each of theplurality of photodetectors is a linear photodetector, the short sectionoptically connected to a first end of the linear photodetector, the longsection optically connected to a second end of the linear photodetector.6. The electro-optic receiver as recited in claim 5, wherein the linearphotodetector is configured to absorb a majority of the first portion ofthe incoming light in a first half of the linear photodetector, andwherein the linear photodetector is configured to absorb a majority ofthe polarization-rotated second portion of the incoming light in asecond half of the linear photodetector.
 7. The electro-optic receiveras recited in claim 6, wherein one or more electrical contactspositioned along the first half of the linear photodetector areelectrically connected to a first photocurrent detection circuit, andwherein one or more electrical contacts positioned along the second halfof the linear photodetector are electrically connected to a secondphotocurrent detection circuit.
 8. An electro-optic receiver,comprising: a polarization splitter and rotator having an optical inputoptically connected to receive incoming light, the polarization splitterand rotator having a first optical output and a second optical output,the polarization splitter and rotator configured to direct a firstportion of the incoming light having a first polarization through thefirst optical output, the polarization splitter and rotator configuredto rotate a polarization of a second portion of the incoming light froma second polarization to the first polarization so that the secondportion of the incoming light is a polarization-rotated second portionof the incoming light, the polarization splitter and rotator configuredto direct the polarization-rotated second portion of the incoming lightthrough the second optical output; a first optical waveguide opticallyconnected to the first optical output of the polarization splitter androtator; a first plurality of ring resonators positioned within anevanescent optical coupling distance of the first optical waveguide,each of the first plurality of ring resonators configured to operate ata respective resonant wavelength, such that the first portion of theincoming light having a wavelength substantially equal to the respectiveresonant wavelength of a given one of the first plurality of ringresonators optically couples into the given one of the first pluralityof ring resonators; a first plurality of output optical waveguidesrespectively positioned within an evanescent optical coupling distanceof the first plurality of ring resonators; a second optical waveguideoptically connected to the second optical output of the polarizationsplitter and rotator; a second plurality of ring resonators positionedwithin an evanescent optical coupling distance of the second opticalwaveguide, each of the second plurality of ring resonators configured tooperate at a respective resonant wavelength, such that thepolarization-rotated second portion of the incoming light having awavelength substantially equal to the respective resonant wavelength ofa given one of the second plurality of ring resonators optically couplesinto the given one of the second plurality of ring resonators; a secondplurality of output optical waveguides respectively positioned within anevanescent optical coupling distance of the second plurality of ringresonators; and a plurality of photodetectors, each of the plurality ofphotodetectors optically connected to receive light from a respectiveone of the first plurality of output optical waveguides and from arespective one of the second plurality of output optical waveguides,wherein the respective one of the first plurality of output opticalwaveguides is optically coupled to one of the first plurality of ringresonators having a given resonant wavelength, and wherein therespective one of the second plurality of output optical waveguides isoptically coupled to one of the second plurality of ring resonatorshaving substantially the same given resonant wavelength.
 9. Theelectro-optic receiver as recited in claim 8, wherein the first opticalwaveguide includes a first section extending from the first opticaloutput of the polarization splitter and rotator to a nearest one of thefirst plurality of ring resonators to the polarization splitter androtator, and wherein the second optical waveguide includes a firstsection extending from the second optical output of the polarizationsplitter and rotator to a nearest one of the second plurality of ringresonators to the polarization splitter and rotator, wherein either thefirst section of the first optical waveguide is longer than the firstsection of the second optical waveguide or the first section of thesecond optical waveguide is longer than the first section of the firstoptical waveguide.
 10. The electro-optic receiver as recited in claim 9,wherein a length of the first section of the first optical waveguide anda length of the first section of the second optical waveguide aredefined to reduce a difference in arrival time of the first portion ofthe incoming light and the polarization-rotated second portion of theincoming light at a closest one of the plurality of photodetectors tothe polarization splitter and rotator.
 11. The electro-optic receiver asrecited in claim 8, wherein each of the plurality of photodetectors is alinear photodetector having a first end optically connected to therespective one of the first plurality of output optical waveguides and asecond end optically connected to the respective one of the secondplurality of output optical waveguides.
 12. The electro-optic receiveras recited in claim 11, wherein the linear photodetector is configuredto absorb a majority of the first portion of the incoming light in afirst half of the linear photodetector, and wherein the linearphotodetector is configured to absorb a majority of thepolarization-rotated second portion of the incoming light in a secondhalf of the linear photodetector.
 13. The electro-optic receiver asrecited in claim 12, wherein one or more electrical contacts arepositioned along the first half of the linear photodetector and areelectrically connected to a first photocurrent detection circuit, andwherein one or more electrical contacts are positioned along the secondhalf of the linear photodetector and are electrically connected to asecond photocurrent detection circuit.
 14. An electro-optic receiver,comprising: a polarization splitter and rotator having an optical inputoptically connected to receive incoming light, the polarization splitterand rotator having a first optical output and a second optical output,the polarization splitter and rotator configured to direct a firstportion of the incoming light having a first polarization through thefirst optical output, the polarization splitter and rotator configuredto rotate a polarization of a second portion of the incoming light froma second polarization to the first polarization so that the secondportion of the incoming light is a polarization-rotated second portionof the incoming light, the polarization splitter and rotator configuredto direct a polarization-rotated second portion of the incoming lightthrough the second optical output; a first optical waveguide having afirst end and second end, the first end of the first optical waveguideoptically connected to the first optical output of the polarizationsplitter and rotator; a second optical waveguide having a first end andsecond end, the first end of the second optical waveguide opticallyconnected to the second optical output of the polarization splitter androtator; a two-by-two optical splitter having a first optical inputoptically connected to the second end of the first optical waveguide,the two-by-two optical splitter having a second optical input opticallyconnected to the second end of the second optical waveguide, thetwo-by-two optical splitter having a first optical output and a secondoptical output, the two-by-two optical splitter configured to outputsome of the first portion of the incoming light and some of thepolarization-rotated second portion of the incoming light through eachof the first optical output and the second optical output of thetwo-by-two optical splitter; a third optical waveguide opticallyconnected to the first optical output of the two-by-two opticalsplitter; a first plurality of ring resonators positioned within anevanescent optical coupling distance of the third optical waveguide,each of the first plurality of ring resonators configured to operate ata respective resonant wavelength, such that light having a wavelengthsubstantially equal to the respective resonant wavelength of a given oneof the first plurality of ring resonators optically couples from thethird optical waveguide into the given one of the first plurality ofring resonators; a first plurality of output optical waveguidesrespectively positioned within an evanescent optical coupling distanceof the first plurality of ring resonators; a fourth optical waveguideoptically connected to the second optical output of the two-by-twooptical splitter; a second plurality of ring resonators positionedwithin an evanescent optical coupling distance of the fourth opticalwaveguide, each of the second plurality of ring resonators configured tooperate at a respective resonant wavelength, such that light having awavelength substantially equal to the respective resonant wavelength ofa given one of the second plurality of ring resonators optically couplesfrom the fourth optical waveguide into the given one of the secondplurality of ring resonators; a second plurality of output opticalwaveguides respectively positioned within an evanescent optical couplingdistance of the second plurality of ring resonators; and a plurality ofphotodetectors, each of the plurality of photodetectors opticallyconnected to receive light from a respective one of the first pluralityof output optical waveguides and from a respective one of the secondplurality of output optical waveguides, wherein the respective one ofthe first plurality of output optical waveguides is optically coupled toone of the first plurality of ring resonators having a given resonantwavelength, and wherein the respective one of the second plurality ofoutput optical waveguides is optically coupled to one of the secondplurality of ring resonators having the same given resonant wavelength.15. The electro-optic receiver as recited in claim 14, wherein firstoptical waveguide and the second optical waveguide have differentlengths.
 16. The electro-optic receiver as recited in claim 15, whereina length of the first optical waveguide and a length of the secondoptical waveguide are defined to reduce a difference in arrival time ofthe first portion of the incoming light and the polarization-rotatedsecond portion of the incoming light at the first optical input and thesecond optical input of the two-by-two optical splitter.
 17. Theelectro-optic receiver as recited in claim 15, further comprising: aphase shifter interfaced with a shorter one of the first opticalwaveguide and the second optical waveguide, the phase shifter configuredto apply a controlled amount of shift to a phase of light travelingthrough either the first optical waveguide or the second opticalwaveguide to which the phase shifter is interfaced.
 18. Theelectro-optic receiver as recited in claim 14, wherein each of theplurality of photodetectors is a linear photodetector having a first endoptically connected to the respective one of the first plurality ofoutput optical waveguides and a second end optically connected to therespective one of the second plurality of output optical waveguides. 19.The electro-optic receiver as recited in claim 18, wherein the linearphotodetector is configured to absorb a majority of the first portion ofthe incoming light in a first half of the linear photodetector, andwherein the linear photodetector is configured to absorb a majority ofthe polarization-rotated second portion of the incoming light in asecond half of the linear photodetector.
 20. The electro-optic receiveras recited in claim 19, wherein one or more electrical contacts arepositioned along the first half of the linear photodetector and areelectrically connected to a first photocurrent detection circuit, andwherein one or more electrical contacts are positioned along the secondhalf of the linear photodetector and are electrically connected to asecond photocurrent detection circuit.
 21. The electro-optic receiver asrecited in claim 14, further comprising: a phase shifter interfaced witheither the first optical waveguide or the second optical waveguide, thephase shifter configured to apply a controlled amount of shift to aphase of light traveling through either the first optical waveguide orthe second optical waveguide to which the phase shifter is interfaced.